The Water Is Wide and Somos El Barco, semi-submersible

You see some strange things if you skim-read these threads.

Anyway, I think theleveller makes a good point - even if you're not familiar with The Streets of London, it's a fair bet that some of your audience will be. (Remember My Lovely Horse!)

My advice would be to write your own. Shut the door and just talk through the words out loud, giving them the same amount of emphasis you would when singing (but not too much - you don't want to end up like William Shatner). I'll bet notes will start to come in. You'll probably find once you get started that you are stealing bits of other people's tunes, but that's nothing to worry about - the refrain of the first song I ever wrote was nicked from a song by Carl Corbett, with a bit of Fire and Rain stuck on the end. (Nobody's ever commented on the similarity.) I wouldn't start by trying to steal bits of tune, though.

Digressing slightly, I once changed the tune of a song I'd written because I realised it was a bit too close to a song by another regular at the local folk club. The first time I sang it with the new tune, I realised that the words were heavily indebted to the same song. Eventually I changed the words to get rid of the, er, homage, and changed the tune back.