Rishi Sunak reportedly entered the Cabinet room in which a surprise birthday reception was being held for Boris Johnson because he was due to attend a work meeting that would follow.

That the police issued him with a fixed penalty notice confirms that they are taking a stern view of what constituted a breach of the Covid rules – in terms both of events and attendance.

The decision therefore suggests further fines to come, since the police seem to be proceeding one social gathering at a time (though since they didn’t communicate a rationale for their action yesterday it is hard to be sure).

Conservative MPs must therefore contemplate the prospect of the Prime Minister being served with further notices before or after Parliament returns next week.

They will therefore be on the watch for what the polls say and their colleagues do over the next few days – for resignations from the Government, for example.

But Johnson is in a better place with the Parliamentary Party than only a few weeks ago for three main reasons.  First, Tory MPs are reluctant to remove him while Europe is experiencing its first war between nations since 1945.

Second, and more directly, they don’t want to upend the local election campaign while doing so – presenting candidates, activists and voters with the spectacle of a party in full meltdown.

Finally, the man well placed only a few months to succeed the Prime Minister – namely, Sunak – was out of the running as a successor even before yesterday’s news of his fine: such has been the effect of the non-dom rumpus.

There has also been a sense that Tory activists (and many Conservative voters if yesterday’s snap poll is accurate) feel Johnson is sinned against as well as sinning – by Remain fanatics, a hysterical media, Labour opportunism.

This could be a disastrous misreading of the public mood.  It could be that the Prime Minister will never recover with voters – and that the exhausted Tories, now on their fourth term of government, are doomed to defeat if he stays.

It may also be that there is no alternative leader, given Sunak’s implosion, who could now turn it round for the Conservatives, especially since to their existing factions they would add another: Johnsonites-in-exile.

Nonetheless, Keir Starmer is no Tony Blair, and Labour’s cultural problems with provincial English voters are unresolved – which is why the Opposition has failed so far to sustain a double digit poll lead.

So much for Westminster.  What about elsewhere?  A Tory line seems to be that Johnson and Sunak’s fixed penalty notices are like those issued to Harriet Harman for speeding and Liam Byrne for using his phone while driving.

This misses the point – as do the legal arguments about whether or not the Prime Minister and Chancellor have committed a criminal offence or whether the police simply think they’ve committed a criminal offence.

For just as a mass of voters will have believed that Akshata Murty should have paid all her taxes here, so they will also think that there is no comparison between Covid parties and speeding offences.

Keir Starmer would not be arguing that Johnson and Sunak have broken the law and lied about it were Labour’s focus groups not getting that message.

And that his rhetoric about voters sticking to the rules while the elites broke them is opportunistic doesn’t mean it isn’t true.

It is certainly unfair that the only person to have resigned over the parties didn’t attend any as far as we know – Allegra Stratton.

However, the Opposition leader will find it hard work to convince even Tory dissidents that the Prime Minister deliberately lied to Parliament.

For the latter clearly thinks he didn’t break any rules by wandering into a party he didn’t know was taking place – hence the first part of his statement yesterday, the explanation, which strained against the second part, the apology.

Nonetheless, it is a dire first for Johnson and Sunak to become the first Prime Minister and Chancellor to have been found in breach of the law by the authorities.

That fact raises a terrible question: has Johnson’s authority to pronounce on keeping the law now gone, along with Sunak’s on paying one’s taxes?

For the Conservatives not to be able to campaign effectively against crime – and for upholding the rule of law – would leave them unable to address a core electoral concern.

Should the Prime Minister now resign?  The Tory interest case for him doing so is not particularly convincing, since there is no alternative Conservative leader with proven electoral appeal.

The arguments from principle rather than pragmatism will seem to many to be cut and dried: Prime Ministers and Chancellors can’t be found to have broken the law (though mind you, the courts so find all the time).

For myself, I can’t see that it would be proportionate for Johnson to quit because he didn’t immediately boycott a surprise birthday party, let alone for Sunak to do so.  Others will disagree.

However, that isn’t quite the end of the matter.  At one end of the moral scale, there is a Prime Minister walking into a party he knew nothing about.

At the other, there would be a Prime Minister not only knowing about such parties but actually organising them.  In such an event, Johnson would clearly have to resign.

Somewhere in between, there would be a mass of parties which the Prime Minister may have known nothing about but is accountable for if the doctrine of Ministerial responsibility is to mean anything.

Then again, that ideal is under increasing strain.  For example, should Amber Rudd really have been the fall woman, so to speak, for mistakes made by her civil servants?

Some of those not remotely affiliated to the Conservative Party believe that the Crichel Down doctrine needs reworking – with civil servants having more of a formal say in government and therefore becoming more accountable.

I want to see what Sue Gray’s final report has to say about what actually happened in Number Ten before taking a view on what should happen to whom (for whatever that’s worth).

Which is roughly where most Tory MPs were during the period between the beginning of partygate and the launch of the police investigation, and where some still remain as far as I can see.

“There were failures of leadership and judgment by different parts of No 10 and the Cabinet Office at different times,” Gray said in her interim report – without then saying whose failures these were.

Prime Ministers should quit in the event of a major policy reversal for the government they lead – an event on the scale of the Suez debacle or the pound leaving the exchange rate mechanism.

For better or worse, I can’t see partygate as an event on that scale but, in commissioning Gray, Johnson laid himself open to whatever she concludes, and if her final report finds him personally culpable he will have to go.