Thanks guys and well done DC 5........... I've finally managed to find it through my son and t'Internet........
Confirmed Grob G 520 Egrett 11. Couldn't definitely say whether it was the (only?) 2-seater ( 520T) as didn't get the reg. Not just 50-odd ft wingspan, more like 103ft!
Really interesting and unusual subject to have seen, & (the type) holds a number of world records...............
N-G Jock