Both the tests from Heavyload and OCCT produced no immediate results, ie crashes. Mind you I didn't run them for a very long time. The interesting thing that I did note was that the CPU temperature was running into the mid 60's, although the CPU fan was shown to be working and responding to the loads.
When running Prime95 however, the small FFT test worked OK, but the the inplace large FFT test, see attached screen shot for details, immediate crashes the system.
So I don't think the NAS is an issue, but I am not sure exactly what is. Does the screenshot give a clue?
Thank you for all the assistance in troubleshooting this issue. The crashes continue with NO NAS interaction so its time to buy a few new components. It has really helped to know that the issue wasn't eventually software and therefore the need to spend money clearly identified. You can close this ticket with my thanks.