The gold standard of sterling is long forgotten and now the supremacy of the greenback has been surpassed. The world has a new global currency - airline frequent flyer miles, which have a greater total value than dollars, euros, pounds or yen.

Doled out by airlines to seasoned travellers, they are intended to be redeemed for tickets as a reward for loyalty. But since their invention in 1981, their popularity has spawned its own economy of trading schemes, charitable donations, enthusiasts and scams.

By the end of 2004, almost 14 trillion frequent flyer miles had been accumulated worldwide, worth between 1p and 6p apiece.