Update:
IT LIVES!!!Well, at the moment it does. It's taken me since around 3 this afternoon to get to a point where it looks as if I can type this without the system crashing, which it has three or four times now. Anyway...
After installing the new bits and switching on, I decided to clone the disk that already had W10 on it to the new m.2 ssd (rather than do a clean install) which went ok - although I got the "unable to reach Windows activation servers" message a few times whilst trying to re-activate Windows but it did it eventually - as it was able to boot from the m.2. However, I had to merge and expand the new partitions due to the usual problems of cloning from smaller to larger; ie, unallocated space.
Then after suffering a couple of crashes, I decided that the bios needed updating as I thought that might help stability and it had the new parameters necessary for a future upgrade to W11. And that was the beginning of my biggest problem as the new bios version might have listed my sata drives, but it declared my m.2 drive as "not present".
So, after doing some searching on my mobile, I found out that I had to enable a new item called the 'Compatibility Support Module' (CSM) which helped a lot as my new drive was now present and the PC boots quite happily off it. Btw, I think the crashes are happening because the Radeon drivers that are on the drive are incompatible with the X570 motherboard that's now in the PC, though the drivers, and the Catalyst software, appear to have mysteriously gone missing. On the upside, there hasn't been a crash for about an hour now so hopefully it will all go well from now on.
I've still got all the peripheral bits to put back into various usb ports so that'll probably take me well past midnight before I go to bed, fingers crossed I don't get any more problems.
Thanks again for all your responses.