HE could, through his late Aberdeenshire-born father Ian, play for Scotland, yet David William Donald Cameron often appears a reluctant visitor when, as today, he ventures north of the Border.

The Prime Minister treads very lightly on Scottish soil as if he is conscious of not setting off any political landmines.

Given the low level of popularity for the Tories in Scotland, this might be understandable. Yet it causes frustration among a number of Coalition colleagues. After all, they point out, he is the Prime Minister, not just of England, Wales and Northern Ireland, but of Scotland too.

With the future of the UK at stake, the PM has to play things cannily. His reasonableness strategy on the technical details of the referendum has worked like a dream, say Whitehall insiders; the plan being not to hand any stick to Alex Salmond with which he can beat those snooty imperialists in SW1.

Thus any plan to swap the steady and urbane Michael Moore as Scottish Secretary with the more up-and-at-'em Alistair Carmichael has been quietly ditched. As one minister pointed out sagely: "You can't out-Salmond Salmond."

Timing in politics is everything. With 18 months to go before the day of destiny, Mr Cameron has to pace himself.

The polls show support for the Union. So, the thinking goes, why scare the horses? Mr Cameron will, for example, only agree to a TV head-to-head with Mr Salmond if he has to – should the polls show support for the Union slipping away.

Yet is HMS Cameron about to increase the speed in Scottish waters? There has been a flourish of pro-UK activity, with Theresa May, Ken Clarke, Philip Hammond, Vince Cable, Iain Duncan Smith and Ed Davey as well as Nick Clegg all crossing Hadrian's Wall to defend policy and the Union while highlighting the demerits of a breakaway.

The Treasury has already overseen the publication of the first UK Government policy paper against independence and another is due on currency this month; this is set to see Chancellor George Osborne venture north too.

Today, Mr Cameron will play a major Unionist card by stressing the importance of the defence industry to Scotland in terms of money and jobs. With Mr Salmond emphasising the SNP's opposition to Trident and its multibillion-pound upgrade, this appears to be a deliberate intensification of the debate.

With his first visit to Scotland in six months, Mr Cameron may be about to put on his marching boots and begin the serious stride into battle.