Thank you Saab, yes, that's it !, my HDD is an IDE type - I do remember the large 4 pin power connector.
OK, well I will order one of those and get the machine down from the loft.
Yes, I found Windows 3.1 to be all I needed for creating documents, diagrams - using Microsoft Publisher, and spreadsheets. No connection to the internet, (only a fax modem), so no security issues with sensitive data, and no problems with hackers or malware. (and no stupid paper clip assistant !). I can listen to a CD while doing my accounts or whatever - old school, but most civilised.