Agent 86 - I have no idea what it did, but for a few brief glorious hours it returned my 3 drives and W98 to life!
Last night I actually managed to limp through to a W98 'Safe' mode, but every attempt to reboot further has ended in failure.
I had also an MDSOS warning 'Secondary IDE no 80 pin cable installed', which was odd since it had been fine. I have replaced that cable with a new ATA cable and now get a Windows GPF warning! Still cannot reinstall W98.