Well, Westminster and the surrounding district smells entirely innocuous, as far as I can tell. But in addition to a similar conversation happening on, of all places, Yahoo Answers Australia, I’ve just had a look at my stats and these are the search terms which have sent people here today:
london smells of poo
poo smell in all over london today
london smell of poo
bad smell all over london today
So it’s obviously real, but the only suggestion anyone seems to have made as to the cause is “is it aliens?”. More investigation is required, clearly.