Many thanks Apocalypsee. Very useful tips.
Downloaded Nvidia inspector. I can't control fan speed and I will delay mucking around with overclocking until I've sorted the basics.
Thanks for the tips and leads. I have uninstalled furmark and installed 3DMark, for directX11.3 which I'm running under windows 10 64 bit. First bench mark I got was about 20. Which was better than 0% of other people! FPS: 0.5 in one of the tests!
During the benchmark the temp got up to 105C! I got hold of some thermal paste, and with a tutorial from you tube on how to dismantle my laptop I was able to reapply the thermal paste to the CPU and GPUs, and successfully reassembled laptop into functional state! That's a win for a start! New temp during 3Dmark test was stably oscillating between 95 and 96C. Didn 't go above 96C. I suspect there is some throttling going on. FPS was about 10 (which is a HUGE improvement on 0.5, and I got a bench mark of 1085. Which is in the 8% percentile.
I've also ordered a new battery. The voltage (VDDC) seemed to oscillate between 0.82 and 0.9. I will report again when I have the new battery. I suspect that a more stable power supply will help.
I'm not sure that going straight into overclocking from here is the right way forward. Recently I had a clean up and got rid of a load of services that I thought were bloatware. Some of them were factory installed, and after that the graphics issues began. I thought I'd postively Id'd every service online before I stopped it but maybe some were essential after all. Maybe I need to do a complete strip and reinstall of the PC with latest drivers etc. I should also speak to the manufacturers to see if I can get the low down on what factory installed services might be vital and specific for my PC, which is an Asus N75SL.
Any further tips regarding drivers and software ? I'll report back again when I've got new power supply and had a muck about with the services and software.
After that, I'm into overclocking which is a new sport for me...
D