This is an extract from the CAA document; SUMMARY OF THE MEANING OF COMMERCIAL AIR TRANSPORT, PUBLIC TRANSPORT & AERIAL WORK
http://www.caa.co.uk/docs/1428/Summa...009May2010.pdf
6.3 Exception No 3 - Cost sharing (Article 267)
6.3.1 A flight will be a private flight for all purposes if the only payment is a contribution to the direct costs of the flight (not annual costs) otherwise payable by the pilot in command.
This is provided –
(a) no more than four persons (including the pilot) are carried
(b) the pilot pays at least a proportionate share (e.g. if four persons are carried the pilot must pay at least 25% of the direct costs) and
(c) the flight has not been publicised in any way except within the premises of a flying club (in which case all the adult persons being carried in the aircraft must be members of that flying club).
So, I can do a 50/50 with a friend, and let his wife come along free, but they can't pay for the flight and let me go free.
I'm still searching for a definition of the "direct cost" of the flight. Can we include the travel to/from the airfield? Like the landing fee, this is money I would not have spent had I not been making the flight.