I DID IT!
And under 2 hours as well. My watch says 1hr57mins, but I'm still waiting for my official time - I think they put them on the website tonight.

It took 12 minutes to get past the starting line, and I was trapped behind 2 people dressed as gorillas for the first couple of miles.
The painkillers wore off at the 8th mile and my calf started to hurt. By the 12th mile I was going on adrenalin and determination - every time my right heel pushed off the ground my calf was twingeing. The 13th mile was a nightmare - the route went pretty much past the finish line and doubled back quite a lot - very disheartening.

Anyway, I enjoyed it, I loved the atmosphere, the crowds were great, and it was very well organised. I'll definitely do another. I can't actually walk at the moment, so I have my leg up on the sofa with ice on it, wondering how long it'll take before I can stand unsupported again - I think I've torn the muscle.

Thanks for all the tips and encouragement everyone.