For month after month, David Cameron has directed a stream of boorish insults at Ed Miliband during Prime Minister’s Questions. Last Wednesday he chucked a jibe at him about “looking as awkward as when he ate that bacon sandwich”.

Today we shall be spared hearing Cameron tell Miliband he is “useless”, “hopeless” and “unelectable”, for the Prime Minister is in Turkey. But we still face the dismal prospect of having to listen to this kind of thing until next May.

There is something demeaning about watching the Prime Minister treat the Leader of the Opposition with unrelieved contempt. The implication is that this reductive version of politics is what works best with us, the spectators. We too are to be ground down by a joyless war of attrition: overwhelmed by the repetition of the same lifeless taunts.

For here is one of the objections to this type of political campaigning. It is not in harmony with the Prime Minister’s personality, so sounds a bit fake. He is pretending, for the purposes of these encounters, to be a street fighter: a boot boy: David Mellor in the back of a cab. The Conservative leader carries the imposture off quite well, for he is a gifted performer, but one never has the feeling: yes, this works, because it comes from the heart and the speaker really believes it.

Labour of course would like us to think that Cameron is really like that. They yearn to see him as a public school cad: a man never happier than kicking some less privileged opponent who is down. They would like voters to conclude that Cameron has no moral right to rule, because with boundless arrogance he considers himself entitled to trample on his inferiors.

It is no defence here to say that Miliband is not Cameron’s inferior. The point is that Cameron has decided to treat him as an inferior: a man so contemptible that there is no point even giving him a fair hearing.

And that I am quite sure is not what Cameron is like. I have no doubt he was brought up to believe it is wrong to sneer: to express open and dismissive contempt for people one regards as weird or idiotic or misguided.

Manners have always been a difficult (though delightful) subject to write about: ever since Chaucer we have tended to confound the moral and social aspects of the question. But our culture has now become impoverished by the failure to develop, during the 20th century, a idea of manners which commands general acceptance.

We do not know, or at least cannot put into words, what constitutes good behaviour. How are we trying to bring up our children? We want them, no doubt, to be tolerant: to believe in racial and sexual equality. Great moral passion is devoted to these causes.

But as far as I can see (and I would be happy to be proved wrong), the advocates of comprehensive schools, while motivated by a yearning for equality and for all members of the community to be educated together, have never managed to say, in a word or two, what kind of manners, or behaviour, or morality, they wish to inculcate. The ideal remains a bit hazy.

This is a big problem for Cameron. He was brought up in an older tradition, described by his biographers, Francis Elliott and James Hanning, in their account of his time at Heatherdown, the preparatory school to which he was sent at the age of seven:

“The school attached great importance to good manners. One Wednesday in the mid-1970s, the headmaster James Edwards decided that standards were slipping, so after games, instead of giving the boys their customary free time, he stood in the middle of the rugby pitch and made the boys walk around doffing their caps to the corner flags and saying ‘Good afternoon, Sir’. The importance attached to courtesy sprang from deeper values that the school was trying to instil. ‘It was about the ability to get on with people of all backgrounds. The notion of noblesse oblige was very strong, both for the school and David Cameron at home, I think,’ says Rhidian Llewellyn [in Cameron’s time a young teacher at Heatherdown, having himself been a boy there]. Alexander Bathurst, who later became a consultant on leadership, agrees: ‘It was very much small-“c” conservative, with good principles – honesty, enthusiasm, upholding the honour of school, family and friends.’ Dan Wiggin says the school sought to cultivate ‘a sense of duty, Christian moral responsibility and awareness of people around you and how to behave properly.’ The relative lack of academic pressure allowed the inculcation of such values to be something of a priority.”

Cameron was being brought up to be a Christian gentleman: the animating ideal of English public schools in the Victorian era, expressed most forcefully and influentially by Dr Arnold, Headmaster of Rugby School from 1828 to 1841. Such earnest language was derided by Lytton Strachey and won’t now do, but its loss has left a vacuum: a point I wrote about in a piece for Standpoint called “Strange Death of the English Gentleman”.

Traditions are fluid, and the tradition of gentlemanly behaviour still exerts an influence. In some ways it doesn’t matter that we no longer have a name for this: what true gentleman ever went around describing himself as one? What matters is how you behave, and how you treat other people.

A friend of mine who knew the young Tony Blair said the future Prime Minister had an astonishing ability to strike the right note with whoever he was talking to: he was as good with a duchess as a cleaning lady.

One of the things people liked about the early Blair was that he had excellent manners. These included an ability to laugh at himself. Here was a man of moral seriousness, who had taken the unfashionable step of getting confirmed while at Oxford, but who didn’t sound insufferably pious. Blair wooed middle England by sounding reasonable and self-deprecating, not by seeming brutal and self-righteous. In the Commons, he liked, certainly, to entrench himself on the moral high ground, but was also capable of taking the bitterness out of an exchange by making some wry remark at his own expense. In those days, he knew the English only find seriousness tolerable when leavened by mockery.

How easy it is, from the safety of the press gallery, to offer good advice to the Prime Minister. At PMQs, it is easy to make a fool of oneself within three seconds. And Cameron knows the feral beasts of the media will jump with relish on anything that can be presented as a gaffe.

So one can see why he has devised various rules of thumb for himself. Any Labour woman who asks him about anything is treated with politeness or even sympathy: he cannot risk being held up once more as a man who despises women.

But Miliband – except on the rare occasions when the Leader of the Opposition takes a statesmanlike, bipartisan line – is dismissed as useless. After all, the Conservatives are going to tell us during the election campaign that the Labour leader is unfit to be Prime Minister, so why not say it now?

One good reason for Cameron not to say it very often is that people would rather make up their own minds. Few Labour voters will desert the party because the Conservative leader says Miliband is useless. If anything, such attacks might inspire a defiant loyalty to a man written off as the underdog. In 2010, many people voted Labour despite being well aware of their leader’s limitations, and the same is likely to happen in 2015.

Another good reason not to say too often that Miliband is useless is that this diminishes Cameron. In the current terminology, it corrupts his brand. Instead of sounding like someone with whom it could be enjoyable to have a drink, he becomes another boring politician who just wants to slag his rival off.

A third reason not to abuse Miliband all the time is that it becomes very, very dull. Great leaders are not dull.

Our tradition of adversarial politics of course requires a willingness to be extremely rude about the other side. But rudeness benefits from contrast: is in fact ruder if set against a background of good manners: can otherwise degenerate into mere yobbery.

It would be enjoyable sometimes to treat Miliband’s ideas with mock seriousness: to hail him as a distinguished socialist thinker: to acknowledge his good intentions, and lament his absurd detachment from anything which might be called real life. To get Labour MPs laughing at him would inflict deeper wounds than anything Cameron has recently managed.

Cameron at his best is a very good speaker. No one is better at saying sorry: I have just watched again his response in Parliament to the Bloody Sunday report: it was remarkably good.

And Cameron can also be very funny. I recall with pleasure what he said about Boris Johnson at the Spectator Parliamentarian of the Year Awards in 2010:

“Tonight is a night for celebrating The Spectator and Parliament. I think the great thing about The Spectator is your extraordinary heritage, the remarkable figures who sat in the editor’s chair. I’m thinking of people like Iain Macleod, Nigel Lawson and also not forgetting my own particular favourite: we went to the same school, the same university and of course I have a soft spot for him, a man of high intelligence and huge ambition, an irresistible charmer with an enviable head of hair, always bursting with brilliant turns of phrase and bright ideas, yes my kind of political maverick.”

Why, we began to wonder, was Cameron being so generous about Boris, an ambitious former editor of the Spectator who went to Eton and Oxford and is famous for his hair. But then Cameron brought the house down by naming the former editor, and Tory politician, he had in mind: Ian Gilmour.

Cameron followed up this coup by going on: “I’m not quite sure what went wrong for Ian. I suppose he rubbed the Prime Minister up the wrong way and never really recovered [Gilmour was one of the Wets sacked from the Cabinet by Margaret Thatcher in 1981]. Shit happens. Anyway, there’s always the chance of becoming our ambassador to Pristina, I suppose.” Pristina is the capital of Kosovo, and Boris had a few days before referred to “Kosovo-style social cleansing”, which he suggested the cuts in housing benefit might produce in London.

Cameron had worked out how to be astonishingly rude to Boris, while also being a pleasure to listen to. PMQs, his staff might point out, is not the same as an after-dinner speech. But Cameron can be witty when he tries, and should try a bit more often.

An astute observer suggests the problem may have something to do with the fact that Cameron appears to have done no debating either at school or at university. He did not attempt (as Boris did) to win the lasting gratitude of audiences by making them weep with laughter.

In my view, part of the trouble is that Cameron is such a complete professional. This is in many ways a virtue, but also leads him in to the error of conducting his politics in an excessively utilitarian spirit. Every speech must be useful: must serve some purpose and form part of some campaign, which means that between now and May, every time the Prime Minister opens his mouth he will commend Oltep (Our Long-Term Economic Plan) and subordinate himself to George Osborne and Lynton Crosby’s re-election plan.

There is a danger that between now and May, those of us under a professional obligation to listen to this stuff will be driven mad with boredom, while the wider public will become more determined than ever not to listen to a word the Conservative leader says.