Not is in our group, remember the hirer is logging the P1
If they are permitted to be the pilot in command then you have in effect told them that they do not require a check, training or anything else. If you had check requriements then they would have to be complied with before the pilot could be in command of the aircraft.
Having authorised them to be pilot in command of your aircraft then even if you insist on having one of your friends in the right seat, they as pilot in command have total control of how the flight proceeds. Your friend can not in any way prevent them from flying as they please, to where they please because your friend is not pilot in command and have absolutelty no authority to prevent anything on the flight.
Having agreed that they are suitable to operate as pilot in command of the "check flight" then you can not prevent them from being pilot in command on subsequent flights unless their standard has reduced below the level that you accepted as being acceptable - a standard that you did not check!
The "checks" you describe are not checks as nothing is being checked it is simply free flying for a group member.
Comments regarding professional pilots in airlines are not relevant because while a line trainer may not hold an instructor rating, they will be an authorised person under the requirements of the Ops Manual and the AOC and thus approved by the CAA to perform the function.
Pilots in command being required to carry persons on flights is not the same as a professional pilot on a public transport flight.............or is it??????
If there is a correct and credible checking system in place then it will serve a useful purpose. If not then it is simply a waste of time and money.
As for the spitfire, you should remind yourself of the requirements to be met by an instructor before being qualified to train on a different type within a class.
Regards,
DFC