Think I have this sorted now. Plans are formulating for 621 VGS to bring a Viking, and Al at the 621 VGS Historic Flight is co-ordinating to bring one of his machines - probably the T. 21, conveniently from the same location. Should make a good pairing, and quite a sight in the Prescott Paddock.
Anyone got a primary glider to complete the scene?
(T. 21 memory. When I was about 14 in the ATC (1968ish) our ATC squadron and another took two T. 21's to Wembley in support of the Schoolboy International. We drove the two trailers onto the pitch at half time and had a race to see who could rig a glider fastest. Then we did the reverse and raced to de-rig and get off the pitch.)