For private investors, it's all too tempting: a fund sits at the top of the performance tables, and the statistics tell us it has turned £1,000 into £3,000 in the past five years. Investors plunge in, committing all £7,000 of their Isa allowance. But six months later they discover that far from having soared to £8,000 their investment has fallen to just £5,000 - and it is still losing money.