Managed to use the computer last night, not for extended periods though, without a crash/freeze/lockup.
The case is still open and the room fan is blowing merrily into the case. One thing I did do last night, as I started to get suspicious after the mention of software changes, is uninstalled my virus protection and reinstall to the prior to update version, the system restore didn't work again. I'll see how things go tonight.
When I started up the system last night, the CPU was c.46C and the system temp (I noticed last night it's not the motherboard temp) was c.36C. Apparently the machine had been used periodically through the day by the missus.
Thanks to everyone for their continued help, if anyone has any other thoughts it would be great to hear them, and I'll keep you posted for the next couple of days.
Jason.