One in 10 of the union's 49,000 members filled in questionnaires that led the Inland Revenue to repay more than Pounds 70,000.
Robert Hodge, the EIS's accountant, said that taxpayers often forgot to notify a change of circumstances, such as a divorce or family death. Mr Hodge expects further difficulties in assessment following the introduction of self-assessment. He advises that a firm specialising in tax law is more likely to win concessions than an individual teacher.
Paul Green, marketing director for Personal Taxation Services, said: "We have got a team of bloodhounds who go by the rules but are prepared to play the game as strongly and aggressively."
PTS takes 35 per cent, including VAT, of money recovered.