From memory (there was a sort of height map think in a newspaper pullout) the finish is about 30-40 metres lower than the start.
A nice downhill at the start, a bit of an incline coming out of Greenwich going along Creek Road, maybe a slight up going into Surrey Quays, and the underpass at about 23-24 miles (after a nice downhill, but even that can kill your legs after that far!).
These are all pretty minor - not what I'd call bitch hills!