Well, unfortunately something similar happened again, but a different risk/issue.
For the last few weeks my left ear (same one as above, it had recovered) has been wet and problematic, but I set off on a ride to, ironically, buy a better and quieter helmet. When I arrived my left ear was 'deaf' and full of fluid. The hearing itself seems fairly normal (conductive hearing and frequency range the same).
This was different though, that the ear plug was too deep this time, and I think it came into contact with my ear drum, which I suspect is partially ruptured anyway (from years back).
So, after dusting the bike from its winter slumber and only getting a few rides in, and forking out for a nice new expensive lid, I think best to take another break to let my ear recover. Took a couple of weeks before, then it got better, not sure if the things are related but I have been getting 'this' a few times in the last month. I think both times it's some sort of fluid in the ear rather than actual damage as such, but for different causes and it's cumulative damage over time (aka 'getting older'). Sigh.