ALISTAIR Darling, the head of the pro-UK campaign, was the guest speaker at a lunch organised by the Scottish Parliamentary Journalists' Association yesterday.

The timing could not have been better. An ICM poll at the weekend had shown the gap between the two sides in the referendum battle narrowing. It had prompted Scotland's leading polling expert, John Curtice, to declare in print that the No campaign led by Mr Darling "was beginning to look like a campaign in trouble".

As even greater luck would have it, as far as the Holyrood press pack was concerned, Professor Curtice was also due to be a guest at the lunch. Unfortunately, the psephologist sent his apologies in the end. Mr Darling seemed particularly sorry to miss him. "I was rather hoping he would be here," said the former chancellor. "We could have discussed tomorrow's poll."

He was referring to a TNS survey which is published today (but which had been issued to the media and campaign organisations under embargo yesterday) showing very little change in referendum voting intentions compared with the previous month.

The ICM poll put backing for a Yes vote on 39%, up two points compared with last month. Support for staying in the UK fell from 49% to 46% while the don't-knows rose by a single point to 15%.

TNS (which published later but conducted their fieldwork earlier than ICM) had Yes on 28% (down one point); No on 42% (no change); and the undecideds on 30% (up one).

Their different trajectories helped Mr Darling make a point he's made several times before - that between now and September 18 there will be many polls and some will be more encouraging than others for either side.

The findings also prompted wildly different responses from the two campaigns. Mr Darling's campaign director at Better Together, Blair McDougall, declared confidently: "The momentum is clearly with the campaign for Scotland to remain in the UK, with support for separation falling even though it is already below historic levels."

By contrast, Blair Jenkins, the chief executive of pro-independence Yes Scotland, said support for Yes was averaging 44% in this month's polls, excluding the don't-knows.

"The direction of travel is extremely encouraging, and we are confident that we will continue this progress," he insisted.

So what to make of another spate of apparently confusing and contradictory polling evidence? Assessing ICM's findings Mr Curtice said that from the No camp's point of view the race was becoming "all too close for comfort".

If that's the case, the TNS poll may provide a clue as to why. Both surveys were conducted after George Osborne ruled out Alex Salmond's key proposal for an independent Scotland to share the pound with the rest of the UK in a formal currency union. The move cast doubt on the currency an independent Scotland would use. A killer blow for No? It seems not. The currency, found TNS, was the main concern for just 5% of voters.