If you have a few months spare you could have a read of the ANO. Look at this
link under the heading
Public transport and aerial work (Section 1 page 107) for all the bits that cover your question. It specifies the number of passengers, how the flight may and may not be advertised, when 'valuable consideration' can be given/received, etc, etc. It's the offical legal document so don't expect it to make sense on the first read
FFF is, as usual, correct.
D
Edited the link to point to CAA site not old copy on HMSO site