Well, it seems that (as I have read in every book/on every forum) you should ONLY trust your hydrometer in these situtations. My SG has magically droppped from 1057 --> 1049 with very little visual indication that it was doing anything! Although, when I got home last night and checked it again (I just twist the top off, lift it 1/2 inch and peek in), there is about a 1 inch ring of very fine bubbles around the outer edge of the top of the wort.
I reckon I will just let it do it's stuff and see how it goes. I might also buy an S-bend type airlock for my new fermenter as well, as my other one always gives me an acurate indication of when I should be confirming sith SG readings that it is ready to bottle...
Thanks for your advice everyone, I will post the results once/if it's done.