Well, it might be (maybe a retained T4, a 'borrow' from elsewhere etc etc) but it might also be a T5 since Tremblers had re-equipped that year with the F3 variant for which, as I'm sure you know, the T5 was the 2-seat variant. I bow abjectly to your 'sore-thumb' spotting skills if you can tell so certainly.
I wager it's probably a 5.
EDIT: Actually you could be right! (and I would be mightily impressed and would eat my logbook). I have just seen a post elsewhere from 'Salad Fingers' which says
"Most of the season was flown with 9 FMK3s or 8 plus a TMK4 - the T~MK5 arrived in September." so the Jury is out.