Gentlemen
Many thank for your responses and all the info. There's a wealth of useful information there and many points that hadn't occurred to me, not the least a level cruise long enough for a cup of tea! The general concensus seems to be that its worth getting up there, ATC and airspace restrictions permitting, even if the level cruise is of a short duration. 185nm/FL240 as mentioned is a good guideline for shorter flights
I also appreciate the point that in crowded airspace cruise levels may be down to ATC requirements and not greatest economy.
My thanks again. X-plane is as close as I'm likely to get to the real thing - I'm 77 - so its as well to get it as authentic as possible.
Boslandew