Sorry to be pedantic, but I think you have spelt “Seyerston” incorrectly (which could effect your searches) in your first post? I think it should be RAF Syerston which I pass quite often and is still very active as a gliding centre.
I may even have flown in your T21 as I had a number of glider flights at both St Athans & Syerston in the 60’s and 70’s. ...but unfortunately no records to confirm identity.
Good luck with the old girl!