The following thred on the PPL forum has highlighted evidence of a PPL (FI) completing Trial Lessons.
As he has a Flight Instructor rating then he is entitled to give instruction, trial lessons or otherwise, and the instrucrtion will count towards the issue of a licence. As he holds a PPL then he cannot be remunerated, but so long as the aircraft is maintained to public transport standards there is no reason why the candidate cannot pay for the hire of the aircraft.
What is a PPL (FI) supposed to do if they can't teach? There are loads of them at Sherwood Flying School Armstrong Whitworth etc.