I actually solved this, and I know many will immediately know the answer, but for those who don't it saved me the time of a full re-install.
Switched on PC this morning to be greeted by a registry error telling me to boot from startup disk and key SCANREG, which I did. After pressing enter it just hung on 'reading files' (or some such). From the Complete Handbook (p29) I found boot floppy and type scanreg /restore. This did not work either.
Resigned to a full reinstall I noticed some 'fixes' being required so, when I had the option to exit setup, I did. Lo and behold - it booted perfectly first time.
Not being an expert I am just wondering how often we might avoid a full re-install by doing this?