DB6......
I think you will find that a person can sit in on a friends lesson (take the back seat), as long as the lesson is paid for only by the student.... i.e. no cost sharing. And a lesson is "From - To" any lisensed airfield (within reason, Gatwick to Heathrow dosn't work), and providing you have on board a student !! It is possible for a student at Blackbushe who wanted to get to Bristol to visit family etc. to hire his instructor for a lesson which ends at Filton, where he is left and the instructor returns on his own, this is very legal!!!!