Thank you for your helpful and prompt reply. Yes, Windows Task Manager is a good idea for monitoring the calculation. I have run it again and it started well, with the CPU usage increasing from 0% to about 10% over 1 minute while the Memory used grew steadily to about 18 GB (of 64 installed) over about 4 minutes. Over this time in Mathematica the Evaluation -> Abort Evaluation option was in black type. Then quick as a flash the CPU fell to 0 and the memory to only 132 MB. The Abort Evaluation option was then grey.
So it had died on me! I know 25 days is too long for any computer to countenance, but it should not give up after 4 minutes. The PC has been working OK otherwise.
For me there are now two questions : 1) why is my calculation suddenly ceasing, and without warning or explanation, and 2) would it be better if Mathematica itself provided some run-time information to the user on progress, rather than having to look into Windows Task Manager? Please advise.