oh man, don't get me started on that stuff - I've been obsessed with London's lost waterworks for the last few years and it's not really a megalithic subject (it's pre-megalithic in fact) - but I think what you refer to is possibly a spring or tributary. The Fleet is a little further east and is still there deep underground - I visited it a couple of years ago. You can't get rid of a river.

When is someobody going to write a book about megalithic London? We have Boudecea's tomb (in fact possibly an iron age round barrow) and the London Stone (possibly Roman and now boxed into the wall of the Bank of China) and that's it as far as I know....