Yes, that's DSB. Square loops in the prototype Dart 15 at the South Cerney World Gliding Championships in 1965. As he disappeared at the closing ceremony display between the hangars (to land on the sport's field out of sight) Andy Gough was on top of the offices behind the hangar with a pile of scrap boxes to push over for a sound effect. Unfortunately at the critical moment a passer by stood underneath to ask what he was doing.
DSB had a splendid air-to-air photo of his Skylark 2 inverted with Cranditz in the background in his quarters loo.
I watched him from on top of an air raid shelter at MiddletonStGeorge passing by below in the Blue Diamonds solo Hunter.
I endorse your opinion above.
Sorry for the thread creep. Incidentally, the YS-53 prototype was modified from the first production T53B (previously XV451).