can anyone please tell me if you should receive a radio code when u get your

second hand vehicle from main dealer

I don't think one is needed I didn't with mine when I got it in December.I've taken the battery off and the radio was fine when I replaced it.

many thanks

no code is needed

though i believe it was once said that the radio is lock to the vehicle vin number or something

Correct - the radio is coded to other ECUs in the car (not sure about vin specifically, but it may be). So if you take a radio from one car and install it in another otherwise identical car it won't work. And if your radio needs to be replaced for any reason so do various other components to key everything together again, making it an expensive replacement!