My question is (or was) intended to be simple.
No problem, made me rethink it... and IMHO it's the answer I gave in my previous post:
"If the aircraft can maintain the same ground speed at both 500 feet and 50000 feet then journey times should be the same. .... I would have thought (a Tornado) it could maintain a high subsonic ground speed of say 500-550 knots at both 500 feet and 50000 feet."
TBF before someone points it out the realities - some aircraft can't maintain high ground speeds at low altitude, (e.g. airliners,) but will bat along at perhaps 500 knots plus ground speed at high level, other types would struggle flying lower ground speeds at high level ( thin air), but from flying something fast and pointy in a previous life I'd suspect/guess something like a Tornado or similar military fast jet should be OK both high and low.
Considering the Earth and the space above it to be a sphere, the suggestion seems to be that flying higher would be further, but the difference relative to the circumference of the Earth suggest that the difference in distance would be negligible.
Correct, but for most purposes and shorter ranges that difference is generally ignored.