Thank you for your on-going interest.
Your previous answers suggested where to look for the problem, and yesterday my son-in -law (who is very experienced in repairing computers, audio mixing decks, etc) decided to investigate for me.
He confirmed your diagnosis that one of the flexible cones is faulty, with a small split, due I suppose to fatigue stress.
As a temporary measure he has interchanged that strip with the one from the far right octave, where the notes are rarely played.
It works !
I have today located a supplier (Beckets Music) in Southampton and ordered a new set of five flexible strips in order to replace them all, and hopefully have many more hours of enjoyment in playing the keyboard (as well as I am able !).
Your answers to my question were extremely helpful because I was loathe to attempt the work myself, but now that I have seen how it is assembled I shall have confidence to do so again if necessary.
Thank you again for your help.