I can't resist extending this thread. My experience was typical: I went solo in a Kirby Cadet Mk 3 with 615 VGS at RAF Kenley in the summer of 1984 - 700ft launch followed by 3 min circuit.
My question is - when was the last ATC T31 first solo? Vikings were already being introduced in '84 and I think 615 converted the following year.