Tim
yes, that's the one - it shows up at all the Vintage Glider Club international rallies - and there's normally a huge queue to fly (on) it!
There used to be a Grasshopper (or maybe it was an SG 38) in the Bicester hangar when the RAFGSA was still based there. It had an altimeter and ASI fitted (cheat!) but I never actually saw it flown.
I flew a Hols der Teufel primary at the VGC rally in Germany last year but it had a "go faster" nacelle round the pilot so you didn't get the same open-air effect (The owner soared it to over 1000 metres off a winch launch one day!)
But the T21 is still my favourite - but 'kin cold at 5000'