Hi Noel and everyone else,
None of this has even come close to answering my original query - why does photoshop need to write a 52.5Gb temp file just for me to open and save an image 1/6th that size. It is that factor which is slowing everything down and not some add-on or external item.
As for buying some super system just to use photoshop, which is supposedly aimed at everyone (read their minimum requirements), I suspect that the vast majority of people neither need nor can afford to purchase such things - I for one can't and actually have no need for such a system other than to create the occasional panorama.
So I decided to search for a program that would do the job and found Autopano Pro. I downloaded the trial version today and set it recreate the panorama. It aligned 92 images (each 60Mb) in less than one minute and then rendered, colour corrected, removed haze, blended and saved the 93 layer PSB file in exactly one hour.
I assume it took that long because there is a lot of vegetation on the mountain - trees, long grass and bushes - and it was breezy that day, so working out how the blend would take some time.
I assume everything was performed in RAM as no huge temp files were created that I could find, and my system was not locked up during the process.
I still have to get to grips with the program but it looks very promising for just 99 euros.