Ok, I don’t have a great deal to update, but promised I would so here goes:
Gas-Safe performed a basic service. He said that he could take the boiler apart and check each individual bit, but he made it very clear that the boiler is very old, some bits have clearly leaked before, some bits look very worn, and so if he took those bits out, there is a strong possibility that they would break when put back, meaning that my ‘service’ could end up as a £500+ bill, or leave me with a boiler that wasn’t working. On that basis I asked him to just do the basic service, check what he could, obviously check all readings etc, but no more. Strangely, the vibration hasn’t happened for 5 days now, and 36 hours since he did the service, so perhaps whatever it was has somehow gone away!
Given the age of the boiler, my plan now is to add cover to my house insurance if I can, but whether I can or not, see if the boiler makes it through the winter, and next autumn (or before if necessary), invest in a new boiler. At least that way we’ll have piece of mind (and who knows, that might even lower our fuel bills). So I’ll start researching new boilers at the weekend.
Thanks to all who have made the effort to help and advise – much appreciated.