James Gallaghan is dining a half dozen to union leaders at 10 Downing St. tonight and the menu is dominated by a single, unappetizing dish. The prime minister is trying to persuade the union chiefs to accept a third successive year of curbs on pay increases. It is far from clear how this will do down.

Joe Gormley, the moderate president of the militant Miners has already proclaimed that there is "not a cat's chance in hell" of his union's agreeing to a third round.

Clive Jenkins, the leftist chief of the big Scientific and Technical workers has publicly scoffed at government contentions that an uncontrolled return to free collective bargaining would create an industrial jungle.

"I would rather live in a jungle than be undernourished in a zoo," he said.

Britain's key union figure and star guest at tonight's dinner is Jack Jones. He is the retiring chief of the Transport and General workers, Whose 1.9 million members are the single biggest grouping. Jones was the virtual architect of pay curbs No. 1 and No. 2, but he is balking at No. 3.

All over Britain, rank-and-file workers are restive. Two years of pay curbs have cut deeply into their buying power as prices rose faster than wages. The wage limits, moreover, have reduced pay differentials between skilled and unskilled workers, another major grievance.

Gallaghan's prescription for curing Britain's economic ills, however, rests on moderate wage demands. His hopes of a boom spurred by exports depends on holding back labor costs.

The concern with labor costs is not unique to Britain, but has spread throught the industrialized West.

Europeans have been dismissing as a "toothless jawbone" President Carter's plan to call in labor and industry leaders for talks on wages and prices. But at the very least, Washington's venture is an implicit recognition that these matters cannot be left to imperfect market forces.

British unions have won a reputation for cranky obduracy. It is astonishing how faithfully they have adhered to the pay restraints of the past two years. In the uncurbed 12 months before the first limit was introduced, wages exploded by 26 per cent and prices hovered near a 30 per cent rate of increase.

Starring in August 1975, the Labir government and unions hammeredout a 10 per cent pay curb "contract." In the year that followed, no union bargained for more, and total earnings rose only 14 per cent. Last summer, a 4.5 per cent limit was agreed and again there has been no breach. Wages have been rising this year at an annual rate of only 8 per cent.

The differences between the agreed limit and the actual result is "drift - the payment of more overtime, upgrading workers within a company and moves to better-paid jobs in other companies.

The high inflation rate of 197-75 was cut in half the next year. This year, however the magic has not worked as well. The plunge in the pound has driven up the cost of imports and consumer prices. So the current inflation rate is about 17 per cent, more than double the pay gains and the source of deep worker discontent.

The pound is now more likely to rise than fall thanks to the inflow of North Sea oil. But the workers still feel ill used and are putting pressure on their leaders who dine at 10 Downing St.

Callaghan's chancellor of the exchequer, Denis Healy, is hoping once again to capture union assent with a tax cut of $1.6 billion conditioned on a third round of pay curbs. He pulled off his trick last year, but he is having trouble repeating it.

One big reason his bait is not being swallowed is because he also imposed stiff new taxes on gas and autos. Many British workers have cars, and the new levies loom larger than the promised cut in income taxes.

There may be a warning here for President Carter and his new energy program. Gunnar Myrdal, the noted Swedish economist, once observed to a reporter, "The most powerful lobby in the Western world is the lobby of automobile owners." That unofficial lobby is now nibbling apart Healey's tax bait.