CLINCHING evidence that there has been a huge net flow of funds from Scotland to the Treasury since 1979, came in an answer from the Government in the final hours of the old Parliament last Friday, the SNP will reveal today.

Not only do the latest figures destroy the last main argument against the suggestion that Scotland paid #27bn more than was received in public spending, they suggest that the actual figure was nearer to #31bn.

Mr William Waldegrave, Chief Secretary to the Treasury, has been forced to concede figures in Commons questioning in recent months, which show that if Scotland's share of North Sea revenues had been allocated since 1979, then the net flow in favour of the Treasury from north of the Border ran to #27bn - a figure which the SNP used to refute previous claims that Scotland was subsidised.

As soon as Mr Waldegrave saw the implications of the figures he had released in January, he attempted to backtrack, and Tories in Scotland fell back on trying to question one key figure - Scotland's share of the UK deficit. This was 17.9% in 1994-95, almost double the per capita share, and disputed by the SNP.

But Scottish Secretary Michael Forsyth called the assumption that this figure of 17.9% was constant over the 18 years a ''ludicrous assumption'' which ''hugely distorts calculations'', and his objection was picked up by Right-wing commentators, and even by Labour campaign co-ordinator Henry McLeish, who described it as a ''heroic assumption, a fundamental flaw''.

But last Friday, as MPs were leaving Westminster - some for the last time - a final written answer to a question from Mr Andrew Welsh, SNP MP for Angus East, emerged.

Mr Waldegrave gave the figure for Scotland's deficit share for every year since 1979, and the average turned out to be almost exactly the 17.9% first identified.

A jubilant Alex Salmond said last night: ''The Treasury answer - wrung out of it on the very last day of Parliament, and after a month's delay - has blown the last shreds of the Tory subsidy myth out of the water.

''For the second time, William Waldegrave has been caught out telling the truth. This new Waldegrave admission proves beyond doubt that it is Scotland which subsidised the rest of the UK - not the other way round.''

He claimed the Scottish subsidy to London now worked out at #6200 for every man, woman, and child in Scotland. The same Treasury analysis, showing an upturn in oil and gas revenues, shows a projected surplus over the next five years of a further #12.5bn.

Now that that key figure disputed by the Conservatives has been shown to be accurate, the only other line of attack for critics of the SNP analysis will be to dispute Scotland's share of oil and gas revenues, and only last week, the Government suggested that the North Sea belonged to a ''region'' of its own, the Continental Shelf, rather than to Scotland or England.

However, Aberdeen University oil economist Professor Alex Kemp, a member of the Scottish Secretary's panel of economic experts, said last night: ''This is clearly not very sensible or logical.''