Pobjoy et al
I believe that the RAF Halton Trenchard Museum has a Cadet Mk3 and a Slingsby Primary glider. The airfield has 2x Sedburghs (1x Silver and 1x Rasperry Ripple) plus a Tost Winch on a Bedford 3 Tonner. No need for a clandestine operation - the equipment is already there!!!
Don't forget that Halton is the current 'Home for Sport' for RAF gliding and that the first UK Soaring glider pilot in 1930 was an instructor at Halton (ex RFC aircrew in WWI). They even have a RAFGSA owned Grob 109b (an unmolested Vigilant!) that flies most weekends to the civilian airworthiness standard.
Best
LJ