Hi Kev and ES.
I've tried cleaning the contacts using the method indicated but that doesn't seem to have made much difference. I do have a cartridge circuit board (without a case) so I can measure the connections.
Re the videos, I think my unit must be the earlier kind as there are two separate boards. And alas neither video showed how to get the plastic 'surround' off the contacts. I'll have a look tomorrow though to see if I can work out a way of getting it off.
Thank you both for the thoughts.
Kinds regards.
From Mike.