I said it seemed very unlikely because it was only about 3 years ago that I replaced them. However, it's not impossible. It also might be possible that the replacement surrounds were never very good (or that I botched the repair) and that the fact that after replacing them I thought things sounded great again was because any bass sounds way better than no bass at all, combined with confirmation bias and my possible inclination to put any remaining imperfection down to the fact that I was never able to set things up properly in the flat I was in at the time.
I have now found the thread in this forum where I came for advice when the original surrounds disintegrated. Unfortunately many posts seem to be missing from the thread including the one where someone gave me a link to the supplier of the surrounds :-(
My week is filling up with other stuff so apologies that I haven't had a chance to try everything that has been suggested yet. I do intend to get to it. I remain grateful for all the input.
Regards,
Simon.