Dr Sarah Ingham is a former Deputy Chairman of Chelsea and Fulham Conservative Association.

Half-past seven on the evening of the General Election is surely a voter’s eleventh hour. Yet, despite a seemingly interminable six-week campaign, let alone five years of knowing exactly when polling day will be, it was astonishing how many people in Twickenham on Thursday evening honestly seemed to have no idea where they’d be putting their X.

The victory of Dr Tania Mathias over Vince Cable was one of the seismic moments of the election. A seemingly rock solid majority of 12,000 was smashed. But apart from a few running the Conservative campaign, no one had seen it coming – least of all, judging by their shell-shocked expressions on our TV screens as the returning officer’s announcement sunk in – the two contestants.

In the last few days we have heard a great deal about a new breed of voter – the Shy Tory. Somehow, hundreds of thousands of these elusive, retiring creatures seem to have been hiding in constituencies throughout the land. They popped up briefly in polling stations before disappearing once more into the shadows. Their preferred habitat seems to be in marginal and Lib Dem seats; they are rarely, if ever, found in Scotland or the Shires.

The Shy Tory, is, however, mythical. If we’re looking for an explanation for what happened on Thursday, we can do worse than turn to one of the Labour Party’s great statesmen: Denis Healey.

What happened in Twickenham was replicated in seat after seat in England, as the Conservatives either saw off a Labour challenge or vanquished a Lib Dem incumbent. Reeling as much as the rest of us from the unexpected Conservative win, pundits and pollsters rapidly attributed the result to those voters who had been too ashamed to own up to the fact that they were actually going to support Team Blue – the so-called Shy Tories.

To the average foot-soldier from a rock solid Conservative seat, Twickenham seemed to have an awful lot of orange placards and posters in its leafy front gardens and bay windows. Cable’s majority seemed impregnable (“inVince-ible”, as someone quipped). A few in the Campaign Centre were reporting unexpectedly positive canvass returns but, to be honest, even during that last weekend the chances of disconnecting The Cable seemed pretty remote for many of the Tory activists buzzing in and out of the office.

Little attention has been paid to the huge number of voters up and down the country who were undecided. They were marked down as “U” on Twickenham’s knocking-up lists on Thursday. When questioned on the doorstep that evening, all the voters who intended to vote, and vote Conservative, confirmed as much. Immediately. Confronted by a blue rosette, most volunteered the information. Their certainty contrasted with a much larger number who were apologetic and rueful, bemused and baffled by their inability to make up their minds. Mathias or Cable? Conservative or LibDem? With less than two hours to go until the polling stations shut, they still genuinely didn’t know. Yes, they were going to vote, but for whom? A lot added that they wouldn’t probably know until they were actually in the booth.

The Shy Tory has been identified by the press and polling companies in the last few days in an effort to excuse their wide-of-the-mark predictions. Those Undecideds in Twickenham on Thursday weren’t Shy Tories, but genuine Don’t Knows.

The best explanation for Thursday’s result comes from Denis Healey – unlike Miliband, genuinely the best Prime Minister the Labour Party never had. In his memoirs, The Time of My Life, he reflects on the result of the 1987 Election. Margaret Thatcher won, he believes, because of the improvement in the economy and the fact that unemployment had begun to fall: in the previous 12 months, most people in work had seen a significant rise in their living standards. As he points out, those people expected a further rise if the Conservatives won.

There would have been a lot less egg on the faces of many if they had read Healey’s conclusion: ‘I cannot recall any general election in which the Government party has lost, when the bulk of the population thought their condition had improved in the previous year and would continue to improve in the next – particularly if they feared that the Opposition would jeopardise their gains.’

The turn-out on Thursday in Twickenham was 77 per cent, a tribute to all the parties’ campaigning efforts. Tania Mathias was an excellent candidate, but Shy Tories didn’t give her that 2,017 majority: it was all those Undecideds who decided at the very last minute that they trusted the Conservatives with their finances.

If we are not careful, the Shy Tory will migrate from myth and become established in the political landscape.