Left to it's own devices it will probably grow until the lens is unusable. Professional cleaning needed, I suspect.
Usually a dampness related problem so be careful where/how you store your kit.
I realised my other lenses with UV filter don't seem to be affected..
Do you or anyone here know any decent lens cleaning service in london?
This is not good news, I'm afraid. And the spores can spread. You may have to grit your teeth and write it off - hope it's not too valuable! Get an estimate for professional cleaning (won't be cheap), but just a couple of spores left behind...
I'm not an expert in this, so do get proper advice, but here are a couple of articles on the subject:
http://photo.stackexchange.com/ques...ungus-form-in-lenses-and-how-to-get-rid-of-it
http://www.truetex.com/lens_fungus.htm
and there was some talk on TP here:
http://www.talkphotography.co.uk/forums/showthread.php?t=393775