Hi Pacion,
Let me qualify my post! I'm not from London but go there frequently and then I try and cram in as much "good" salsa as I can.

So it's sort-of one traveller's info to another!
Saturday nights are still the night I haven't found the "perfect" class/club. I've only been to Colloseum once, last time I was there (2 weeks ago) - it's a mission to get there, did the Rueda class just for fun 'cause I was there, but had some very enjoyable social dancing afterwards... same with Ronnie Scotts, went there after Colloseum (closer to hotel and on night bus route after tube-time than the Colloseum - important consideration for the traveller!) and again had some nice social dances! Both these places, maybe it was just lucky that I had a nice evening... maybe I ran into some nice dancers by chance... maybe I was just in a good mood

, so can't really vouch for them generally. I know you are 100% right about Colloseum - it's not always on.
As for Salsa! Latin Quarter, Sound, My Place, Jongleurs, Ealing - been quite a few times, same good experience!
As for Fiesta Havana, it depends on the luck of the draw the night. I phoned before I went last time, and they said Sunday was primarily salsa. I've been there on a Thursday that was good, and other times you go and its too crowded or its very Brazilian samba music; but I've also been on a Brazilian night (e.g. Monday) where later on it was very nice! The nice thing about Havana is it stays open quite late and is close to night busses to about anywhere, and, well, I just seem to like the place

But I must add - I normally go there very late; then they play more salsa and it is not that crowded so you can play a bit with steps and stuff on the music...
Just to put things in perspective! But again, there is so much Salsa in London, you definitely do not have to sit still any night of the week! Like Bar Cuda in High St Kensington also used to be fun on Wednesdays (and easy to get to), but I haven't been since I discovered Jonleurs in Bow Wharf (which is a bit of a mission to get to). But once you've worked out the times, the tubes, the buses, it's all well worth it!!