Well that was messier than I'd planned. I didn't count on there still being so much coolant left up high after draining the rad
Right, well to answer my own question the seal is 37mm ID, 41mm OD and is currently about 1.2mm thick. However the groove it sits in is about 1.2 and a smidge millimetres deep
Over the last twenty four years and however many miles, it's baked itself a little dry and shrunk. It may have originally been an o-ring and got absolutely squashed, or it may have been flat to start with. I'll have to browse the Yellow Pages and talk to some seal folks to see if they've anything suitable. Meantime I'll maybe try soaking it in coolant to get a bit of life back in it, then pop it in with a smear of Hylomar and see if that does the trick - it's easy enough to trial, in the grand scheme of things ...