Apologies for not responding earlier -- I was in Cambridge most of yesterday, and did not have time to deal with people's replies until today.
Many thanks to all who responded. I do seem to have raised a lot of dust in attics!
Since I was in Cambridge, I followed up Ken's suggestion, so popped into Maplin's and bought an L90BT (PS2 female to DIN male adaptor). As Wayne indicated, it wasn't cheap (£7.99), but is nicely made and solid. Since Maplins had a few "on the hook" in the shop, there was no issue about being "lucky to find one in a hurry"!
Anyway, back at home, this morning I used the adaptor to connect a PS2 keyboard to the DIN socket on the box. Worked perfectly, and I fscked the filesystem (Statutory Declaration: I have not made a typo). I am now back to emailing normally from that machine!
The following may be of interest (and a possible useful warning). I dismantled the old DIN keyboard to try to diagnose the problem. The heart of its mechanism is two plastic sheets with "printed circuitry" on them: on the underside of the upper sheet, and on the upper side of the bottom sheet. These lie below the keys, and when a key is pressed it presses the upper sheet down onto the lower sheet, and makes a contact. I suspect that this may be the source of the problem.
Because the computer in question is mainly accessed remotely, its keyboard is hardly ever used (except occasionally when rebooting); so, to keep the keyboard out of the way, I have had it standing upright on its edge (the edge that would be nearest to you when laid flat for use). Undoubtedly, at least when putting it back on its edge after use and probably often when just shifting stuff around, the keyboard has been tapped onto the tabletop when in the upright position. This may have caused the plastic sheets to shift inside the keyboard, either getting it out of alignment with the pegs on the keys (that press down on the sheets) or moving it out of touch with the contacts in the keyboard case that connect the sheets with the little chip that feeds the cable. Whether that is the explanation or not, the end effect has been that pressing a key produced no result (and the effect had been intermittent until yesterday, when it had apparently become permanent). So beware!
(Another warning: dismantling a keyboard should not be done lightly. There are lots of loose bits inside. Reassembling and getting everything back in line is not trivial. Indeed, I recall, from years ago, taking the bottom off a keyboard only to discover that it had spring-loaded insides. It took ages to pick all the little bits up from the floor ... ).
I shall be in touch privately with the people who have kindly invaded their attics on my behalf, once I have brought that machine properly up-to-date again.
Meanwhile, thanks again and best wishes to all, Ted.
-------------------------------------------------------------------- E-Mail: (Ted Harding) Ted.Harding@manchester.ac.uk Fax-to-email: +44 (0)870 094 0861 Date: 28-Jan-10 Time: 08:57:50 ------------------------------ XFMail ------------------------------