I'm unable to find any ruling prohibiting a passenger having a go with the controls.
I have a feeling that it's permitted "within reason" ie the flight isn't allowed to be logged as an instructional flight, the pilot doesn't accept payment for tuition unless they have an F.I rating. The passenger doesn't operate the radio except to make a mayday call if the pilot is incapacitated.
I remember actually seeing a couple of books either in the Flying Shop catalogue or the Transair catalogue which are purely for the enthuastic passenger, this was a while ago and to be honest I've not really looked again.
As long as the pilot is in command of the aircraft at all times and the passenger is not doing anything that could endanger the safety of the aircraft or the other people in the aircraft, I think it's one of those things that the CAA will probably say "well if you don't do anythng drastically wrong, then we won't say anything".
Had I looked at the forums last week in more detail I could have asked this question as I was at the CAA on Thursday.