Michael Settle: "Why are the Tories looking so pleased with themselves?"

First Minister Alex Salmond yesterday posted an invoice to Downing Street for £500m, and that's just for starters.

He reckons that is 10% of the windfall going to Chancellor Alistair Darling since basing his Budget on much lower oil prices than now prevail.

The SNP leader hopes to get three times as much, if Holyrood receives Scotland's population share of all this year's North Sea revenue.

The intention is to sound reasonable, while establishing the principle that Scotland deserves a share of North Sea oil revenue. Nationalists hope that could mean 90% of offshore tax take if the nation becomes independent.

The intention is also to use the money to start an oil fund, similar to Norway's, so that Mr Salmond's talk of "support for hard-pressed sectors of the economy, such as ferries, fishermen and public transport" requires patience, as these would have to wait until the fund starts delivering significant dividends.

The claim on oil revenue is easily dismissed by Mr Darling. Scotland's share of Treasury spend, much of which does not come through Holyrood, already benefits from North Sea revenue.

But with Mr Salmond joined by Labour back benchers clamouring for a swift response to fast-rising energy costs, the Chancellor's response could put him in dangerous political territory.

He faces three dilemmas. The first comes with the argument that rising oil revenue is necessary to counteract the impact in falling tax take from other sources. In a TV interview broadcast yesterday, Mr Darling said stamp duty revenue is sure to fall as the property market slows.

But if he uses that argument, he has to admit his other tax plans are in shreds. Economic growth projections made last autumn have been overtaken by global inflation and domestic political pressure.

Along with stamp duty, a slowdown means less income, corporation and value added tax, and reduced revenue from duties such as alcohol. Recession would mean the unemployment welfare bill rises, public sector workers demand their wages match inflation, and hiked energy costs means more cash needed to run buses and heat hospitals.

The second dilemma is balancing the need to look tough in the face of adversity without seeming deaf to public pressure. Having already conjured £2.7bn out of borrowing this year, to bail the government out of its 10p tax band debacle, it won't look impressive if Mr Darling is buffeted into further concessions throughout the tax year.

Postponement of the intended 2p per litre fuel tax accelerator this autumn is now a given, though Mr Darling cannot yet allow himself to say as much. Hauliers have already sensed previous capitulation has given them added leverage, and if they get the 2p postponed, they could press harder for a 25p concession. According to Mr Darling, he cannot respond to intense public pressure before his pre-Budget statement, and that is at least five long months away.

The third dilemma affects all parties: reducing tax on oil runs counter to their commitments to reduce carbon emissions. Concern for the planet is being jettisoned as economic times toughen. The SNP also has to be careful of arguing that Scotland deserves cheap fuel because it is oil rich. That tack previously won it a reputation for "toxic tartanry" that it fought hard to shed.

Mr Darling's BBC Scotland interview yesterday also focused on several of the fronts on which he is battling the SNP administration. He attacked the SNP opposition to new nuclear power plants, saying it would be "daft" to leave Scottish households dependent on English power generation, while criticising the reluctance to allow new wind-farm developments. He repeated his attacks on the SNP's local income tax plans as "daft and damaging", but did not rule out Westminster permission for Holyrood to use the Treasury's tax-collecting agency, HM Revenue and Customs, to collect the tax. He claimed SNP ministers had not asked, or spelled out how they want the new tax to work, alleging his opponents wanted to avoid discussions about it.

Scottish Finance Secretary John Swinney responded: "It is astonishing that Alistair Darling doesn't recognise there is a massive windfall from North Sea oil revenues with the price of oil hitting such high levels.

"He is living in a fantasy land if he believes that and is about to become engulfed in a fuel price fury."