Final, final update.
On impulse I sent a message to Schaller this evening using their webform. I got a reply a short while later from Dr Lars Bunning who is the Schaller managing director and owner asking me to send a picture of the damaged machine head and he would try to source a spare part.
This is stunning service from Schaller in trying to support a part purchased over 28 years ago.
It has completely made my day.
John