Thanks for the replies.
Michael McNam - strange as it might sound, my knee first went about 14 years ago when I sat down awkwardly from standing - suddently it wouldn't support any weight at all. From then on it would suddently 'go' from time to time. They sent me for an X-ray and nothing showed up. The doctor never suggested a Plan B (bizarre in retrospect) and as over time it only 'went' infrequently, I learned to live with it.
About 3 months ago it started to get much worse (I started running in the gym at this time). 1 month ago it deteriorated further and has swollen up.
Amazingly, aikido (I've been training for two years) never aggravated it at all (seiza, shikko and all), though obviously I've had to put it on hold now, until I'm sorted out.
Michael H - thanks, that's really useful stuff, though watching my peers from off the mat for at least several weeks will be frustrating. Useful point about telling the doc - the physiotherapist I was initially referred to had never heard of aikido! Thankfully, there is much to engage oneself with regarding studying aikido off the mat- even with injuries
Sounds like a degenerative meniscus rather than a true trauma (although it was probably traumatic initially).
These can take longer to recover post-surgery so dont rush and do the physio.
One other thing I don't know you're age but as you get older recovery takes longer.