noggin - I've read the relevant AIC on this (can't remember which number) and this seems to imply that even if it a soley owned aircraft you can only pay for instruction given to add a rating to your licence or for tests, revalidations etc. i.e. not for either PPL or CPL instruction. Is that the way you read it?
Of course, even if that is the case, there's nothing to stop an instructor teaching you on your own aircraft for free and then charging you £1000 for 1 flight in a plane on a public C of A.