Fixed it in about 45 minutes. There was an Ebay link in previous wiper earth threads that I used to buy a ~50cm-long thick earth cable with 8mm eyes at the ends maybe 3 years ago and put in a box in the garage in case this happened. Managed to find it, unscrewed a 10mm bolt head on the wiper motor casing next to where the wiper crank comes out, sanded the casing surface underneath, replaced the bolt with the eye and a washer securing, then managed to run the other end to one of the three bolts securing the alternator belt tensioner. Turned the ignition key and wiper springs back into life.Enjoyed my other half discovering the windscreen wiper earth issue - in my car - .. (my first ever experience of this, this car now sitting on the drive in torrential rain). I've checked with jump leads and the wipers work fine but not without the extra connection.
So now she's got her car I've been nursing with the non-working oil pressure sensor issue instead (occurred during lambda replacement described elsewhere, but manifests as a loud bong, red oilcan symbol intermittently when driving). Also intermittent symptoms of brakelight switch that I was going to try to replace this weekend. This is going to be a fun day.
Now onwards!
Thanks to all here who have commented on this issue and potential solution over the years ; should probably have fitted the cable in the summer. Easy fix and one I'm going to repeat on the other cars as a preventative measure.