Sir William Pile may well be owed £ 3.000-minion-so he disclosed as Chairman et the Board of Inland Revenue earlier this year. Many would argue that hi& estimate of debtors is modest. Moonlighting, the dodges of the self-employed, tax ' plans' for top people, and the activities of giant multi-nationals, all contribute to a growing sector of Britain's finance - the black economy. To the overworked taxman, it might appear at times a losing battle. But who is the real loser - the country, the honest tax-payer, or those confined by paye? In the second of an investigative series David Perrin surveys the battlefield with the help of experts - and practitioners. Producer RITCHIE COGAN