That's what mine thought at first, until he took it to the garage and they said its one of the most common problems they have to fix. On the vespa its really easy to just pull out the fuel line and drain the tank. Its just the 'considerate' thieves that put the line back afterwards and don't cut it - in which case it is a tenner of fuel and 30 quid for a new fuel line! STG, where in SE23 are you?