I bet you've been messing with the exhaust and jetting? Stock exhaust, stock jetting, 1 extra tooth on the gearbox, nice bit of French N road and it turned in a steady 60-62 mpg.
I've also seen under 25 mpg on either the Dragon or one of the llanthony runs, which proves that running to reserve and having to buy petrol in poxy village garages is a bad idea.
Andy