I have a Ducato motorhome and it seems the fuel cap is an attractive item to local thieves. locking cap that can be twisted off is not good. The filler neck only has the capacity to "catch" at the bottom edge of the filler pipe. ?This design can never have been secure and I doubt conforms to the UK C&R Regs. Has anyone else ever experienced a similar problem and if so what is the answer? Is it a new tank??

Are you buying the correct fuel cap because on mine when I lock it it spins around till I unlock it again, have a good look at the filler to see if it bend or out of shape and the cap is not locking properly to the neck, and it might be that the cap is falling off and not being stolen as you say. Good luck m8

Yes I am buying the correct cap it does spin when locked. My problem is that the filler neck only has one retainer for the cap to locate behind to prevent it coming off. ~A sharp tug downwards on the cap and it will come away from the neck. Cheap and nasty!!!

They're hardly doing it for the monetary value unless they're also siphoning the diesel so if you they don't get a chap vandalism fix they may move onto your wing mirror etc instead. How about fitting a magnet and reed switch alarm to the filler flap.

Thats a good idea.! Thanks for that. Have to locate a switch now. They are not nicking the diesel. That's the surprising bit about it but the cap can't fall off even if it comes unscrewed as the flap soring is quite good. Maybe fitting a lock to the flap is the ideal solution.