Tourism is Britain's largest dollar earner. Our culture and heritage are one of our most important invisible exports. Although the number of Australians and Europeans coming to England rises every year, the majority of visitors are American, in search, perhaps, of a past they feel does not exist in their own country. What they want to see is Changing the Guard, Stratford, a couple of castles and with any luck, a living, breathing earl or marquis. Jeremy James has been looking at the new tourist boom - at the package tourists who do Europe in a fortnight and Britain in three days, and can go off the beaten track with a stage coach and a country weekend.