Thanks for the info everyone.
Chilli, how should I let you know that it's just a transit flight? Would the initial call of "Blah Approach G-ABCD request Flight Information Service" let you know or should I be more explicit and say something like "Blah Approach G-ABCD Transit" or something similar?
I always PPR when going somewhere if it says so in my VFR flight guide - it's dead easy to do and it's also a way of getting important information like whether AVGAS is available. Sometimes the reply seems to be along the lines of 'Yeah, course you can come - why are you ringing up and asking!", but I don't mind that.