Thank you for your welcome comments; I don't feel so alone now. To answer your questions:- My Mac is a 15 inch Retina, early 2013, 2.7 GHz, Intel Core i7, 16 GB 1600 MHz DDR3, Intel HD Graphics 4000 1536 MB.
I tried downgrading to Yosemite but had the same 'red heat'. I'm back on El Capitan and still 'red hot'. Apple in Watford subjected my laptop to a 45 hr hardware test and all is normal. I downloaded the Apple Temperature Gauge for a few pounds just to get some proper evidence and saw 105 deg C once or twice. I downloaded Unigine Valley bench-tester (free and suggested by Apple Support). That test roasts my Mac too. Fascinating that your mother's machine performs flawlessly; is it a laptop too?
The solution I have come up with is to sit my Mac directly on a hot-water bottle that's been in the freezer for several hours. This gives adequate cooling provided that I occasionally move the Mac around a bit if the temperature gauge shows the Mac is getting extra hot. If the temperature is acceptable i.e. up to about 85 deg C (guessing) you see green bars only, associated with each of the CPUs. I normally have to put up with one or two amber bars. If I get a red bar, I have to panic and move the Mac quickly around quite a bit to lose some of the heat. After all the hours of phoning and visiting Apple and generally thinking of a solution, I am pretty sure that laptops are so brilliantly made in such a small volume that the fans and exit holes are just not designed for X-Plane 10. This software does so many millions? of calculations per second that it produces too much heat for our machines. You may know that 'gaming machines' have water-cooled' video cards. I've seen them in Maplins
PLEASE let me have your comments. Regards, Roger Adamson.