You must be right - around 95mph. The speed of the wheels revolving in the filming from the aeroplane, at around 1.00 can be compared to these modern shots of the Gresley A4 pacific doing a certified 90mph. The A4s driving wheels are 6'8" in diameter; the Castle class wheels are just half an inch larger. kzfaq.info/get/bejne/o9R1g7F9yK3DoYU.html There has been a handful of occasions during the 1950s and early '60s when a Castle class engine was timed reliably at over 100mph. But for various reasons, the GWR never had a stretch of line where a speed of 100mph-plus could be sustained for several miles on end, which the LMS and (especially) the LNER had during th 1930s.

I don't think this is the 81.68 run I think this is an earlier run with no 5000 I'll have a look at my books to make sure. Also I think I can see 5000 as the loco number at 1:09 but I can't be sure.

​@@thomasgray4188If you guys want more detail, the driver wrote a book about it - and his career - in 1951 called 'I Drove The Cheltenham Flyer'. There's a few copies of it knocking around here and there.