Start button does not operate. Keeps flashing and I cannot turn it off. Have taken side panel off and find that the main fan is only turning one eighth of a revolution. This seems to be connected to the start button, so could this be the problem?

That could be several different things. It could be the power button itself (not so likely) or it could be related to problems with the PSU (power supply - and more likely than the power switch). It could also be a bad motherboard but that's a lot less likely than the PSU. In many years of working with and on computers, I've only ever once had a problem where the cause was the power switch itself. Especially on the modern ATX form factor where the power switch isn't an on/off switch like the older AT form factor, but a toggle. If you have access to another PSU (in good working order), I highly recommend you try it before you test anything else.

many thanks to CurtR for informative and helpful reply. Will certainly change PSU and see what happens. In the meantime I got computer started by switching on and off at the wall switch.