Thank you for everyones replies - appreciated. Bits of everyones suggestion was the solution.
I have to go back out again in a bit, but wanted to update where I am with this so far.
Id already tried booting into safe mode but that just seemed to hang at some point, unless it was without network... so it was obviously something to do with networking.
I also discovered that if I tried to boot with a LAN cable plugged in, I got the black screens. Nothing would happen at all. Ive no idea how/why a LAN cable was plugged in or not would even stop everything, but this could be repeated. CAble in and black screen.
Without LAN and windows would eventually (but slowly) load. So I removed the intel NIC. Windows loaded. I deleted the onboad LAN drivers and let windows re-discover it. Now I had networking again. I went to shut down and it took ages, but it now it seems ok. PC is still on the floor atm doing a scan-disk to make sure all is ok with the HDD.
Not tried putting the Intel NIC back in again. But a quick speedtest shows I still have the slow throughput issue with the onboard LAN.
One of the error messages I got whilst trying to load windows was the black/white hardware changes one, with the option to last good known config etc.