Thanks for all the suggestions.
It is definitely leaking from where the thermostat housing pushes into the waterpump, bolt is done up, bore in waterpump is not scratched & no obvious reason why it should leak, new hoses with all fastenings done up tight. Have fitted slightly oversized nitrile O ring using silicone lubricant. In short everything suggested.
I guess the next step, which is a bit radical given the engine has only done 4,500 miles, is to replace the waterpump housing but before I do that I'll try another O ring (that will be 5).