Frank Costin was Mechta Senior's gliding instructor at Portsmouth in the war, when the latter was an air cadet. There was still a civil gliding club operating there and they shared aircraft, so the air cadets were some of the few who had access to a two seater glider for training. Presumably Frank Costin was working at Airspeed, who were on the same airfield?
The housebuilding thing must have rubbed off, as Mechta Senior designed and built two in his spare time. The second having a basement designed to accomodate the full span of a 15 metre glider wing. The house got built, but not the glider!