Arc,
Very interested to read your description - I also flew with 631 VGS at Sealand, but back in the early 60s, so I didn't care to comment on its suitability - but it sounds as though little has changed in the intervening years!!
Mind you, I do remember getting 'wave lift' from the Welsh hills, augmented by rising heat and fumes from the nearby John Summers steel works - ah the smell!!