Daifly, it is acceptable (to the CAA) to carry a 'passenger' in the back on a 'trial flying lesson' (although in principle there is no such thing as a trial lesson) as long as the person carried will gain from the flight. This boils down to the fact that the person in the back may have been the one who gave the voucher to the student in the front as a birthday gift etc. Another reason would be that maybe they are themselves considering taking lessons at a later date. All that has to be proved is that the 'passenger' is involved in some way with the flight. Hope this helps a bit.