Thanks for that response David. I wonder if the cause is to be found on my machine rather than in Lilypond.
I have recently been looking at Python (to try and stimulate a few aged grey cells) to which end I have blundered about with Trinket and also installed Python3.8 (64bit). These efforts have stalled since I have not found a way of including "numpy" and "processing" outside of Trinket. Could this action in anyway interfere with the Python part of the Lilypond installation? My problem did not appear after any Lilypond update so very unlikely to be Lilypond related. In an earlier life using a remote mainframe I stumbled on a freak problem. The secure network had a series of PO repeaters. These repeaters could all be controlled centrally by the PO using signals (not character data I believe it was frequency modulated signalling). It transpired that my data contained one sequence of characters that were interpreted by one faulty repeater as a shut down command. The repeater worked correctly for 99.99999% of the time for a number of sites until it met my innocuous string. You can imagine how hard it was to convince all the technicians along the line that the fault was in the line somewhere not my input since no other users (there were a lot!) were affected. Some issues can be very hard to resolve. Regards Peter
