This is the point that I stand up in the middle of the group and say "Hi, I'm Alison and I'm a complete and utter plonker"
It's all working, up and running.
Stupid here hadn't got the new heatsink when I'd put the new processor in so I tried with the old one (which didn't cover the GPU), and it was switching off because the GPU was overheating majorly.
New heatsink arrived, fitted and it's working tickety boo.
Massive massive thanks for all the help.
There's quite a visible gap between the heatsink and the GPU but I'd no idea what thickness pads to get. I phoned Dell and once I'd been sent around the world, twice, eventually got somebody in England who told me that they would just use paste. When I said there was quite a gap, he said just to pack it in!! Don't get me wrong, it's not thick with it, but I put more on than the normal "half a grain of rice" size and it's contacting and seems to be OK. I'll get speedfan on it just so I can see.
I can't believe it's fixed!! I'm gobsmacked!