Darkerside - what were you using. UCI compliant or other?
My own fastest was off Canick Chase moor hill following Ian F on his modified / faired Zephyr very low racer. This used correx to improve the aero. The empty road was so straight and long that I got to 50 on an early mk 1 Fuego. Basically we entered top of hill close on 30 then gravity as I spun out. Ian was using a 60T chainring on his so he could power up to around 40. Think he did 55. I drafted a bit!
On the Stow to Lauder road I'm sure 50 might be possible. There are corners though.