See also the CAA's summary of the position in
http://www.caa.co.uk/docs/122/summar..._transport.pdf
I am always sceptical of the motivation for questions of these kinds from PPL holders, especially those with commercial flying aspirations. As I said on the other "please tell me how to do somewthing dodgy and get away with it" thread that is currently running, it's worth recalling what the first P in PPL stands for. If you want to fly for money, get a CPL.