Have you tried a reverse flush of the cooling system? There are a lot of products to clean out cooling systems as this is a common problem.

Stick to what it says on the packet though, I thought that leaving the cleaning stuff in a bit longer would be a good idea, however I 'flushed' out the seal on my water pump.

If you think that there is a problem with the water pump, then take the thermostat cover of and half fill the system. You can then run the engine (from cold for a short time) and see if the pump is pumping.

It is is possible that the the impeller is damaged.