Finding a GP pronto?

Just heard from work - they want a medical certificate for my week of illness. Trouble is I'm in London, and I'm not registered with a GP here. What's the best way to find a GP pronto to get seen for a certificate?

Update: I'm still ill, though recovering pretty well now. I may be well enough to work by tomorrow, but I expect I'll still be able to produce enough barrels of mucus to satisfy any GP that I was Not A Well Bunny...