I think I only ever went that high once in a Hunter - to do a boom run in a Valley GT6.
We were told that if you used flap above M0.9, the elevators would jackstall and you wouldn't be able to recover until you raised the flaps again...
We never bothered using the follow-up tailplane system either.
Drove back from British North Humberside today along the A15 to Lincoln, then the A46 Fosse Way to the M1 - I see that at last the road is being dualled all the way from Newark to the M1 at Leicester. But a it's a pity to see that the Officers Mess at Syerston seems to have disappeared under part of the new junction at Flintham.
Back in the late '60s on that you route you would have seen RAF aircraft at Scampton, Swinderby, Syerston and Newton. But apart from Air Cadet gliding at Swinderby, sadly they're all gone now.....
