About 3 years ago the Head of Enforcement at the CAA tried to implement a change in the law to stop people being carried on "Trial Lessons" now if it was illegal allready, they would not gave gone to the bother. They gave up, because it was all too complicated and had knock on effects e.g. it would prohibit revenue flights to and from maintenance bases etc and prevent the carriage of two students etc. I have regularly carried someone in the back, when appropriate to do so; I deceide, there is no problem.