LET'S be honest, it's not been a great week for the Prime Minister.

After the Itchy and Scratchy Show - Alistair Darling and Alex Salmond that is, not Alistair Darling and Gordon Brown - the key factor now in the referendum campaign as it moves towards its grand finale is momentum.

Following the TV head to head, which pundits (me excepted) gave to Mr Salmond, eyes would have been on the first poll and it has duly arrived, cutting a 13-point lead for the No campaign to one of just six points. This means a further swing of four points would produce a Yes victory.

The late narrowing reminded me how 18 months ago, three senior Whitehall sources each separately and in quick succession privately declared a No victory was in the bag and all that was up for debate was just how small the Yes vote would be.

This was said in such a nonchalant way at the time my jaw dropped given, in particular, that UK ministers were vigorously insisting in public there was not an ounce of complacency within the Coalition breast.

Dare I humbly suggest today that complacency, albeit privately harboured and expressed by admittedly some and not all Coalition sources, has been the UK Government's Achilles heel and, of course, could, as the Greek hero found to his cost, ultimately be its undoing.

Early on, the Lib-Con Coalition focused its attacks on what it thought were the holes in the Yes camp's Gorgonzola prospectus for independence; that, if it plugged away sufficiently enough, those four in 10 undecided voters would clearly see the dangers and step back from the abyss of separation.

But this was never going to be enough.

There was little appreciation, which still remarkably exists, that there is an equivalence of the propositions from both sides in this debate; that the Coalition has to talk at least, if indeed not more now that time is zipping by, about the merits of Scotland being part of what many regard as the most successful political, social and economic union in world history as it does about the demerits of breaking the historic link with England, Wales and Northern Ireland.

The coincidental exit from the Tory fold of Douglas Carswell to Ukip as David Cameron defended the UK in Scotland was not only an embarrassment to the Conservative cause and the PM personally but it raised yet again the spectre of Europe, which has so haunted the Tories for so long.

But the timing meant it played into the independence debate as the Yes camp helpfully pointed out how they were, in fact, the true Unionists - in the European sense - and warned that if Scotland did not separate from the UK, then the UK would separate from the EU.

Politics is cyclical; what is up one minute is often down the next. But time is now running out. Mr Cameron cannot afford to have another bad week; there are, after all, less than three left. If the Union is to be saved, he and his colleagues need to find some momentum from somewhere and fast.