During the recent work being carried out I asked my mechanic to take a look at the cooling system. He reproduced all the problems I have seen, but apart from a small weep at the thermostat, which needs a new gasket, he could not find the leak.
Unfortunately that means it is probably a head gasket problem. It's not going into the oil, which is good, but the fact that the system pressurises before the engine warms up probably means there is a leak into one of the cylinders. We haven't managed to do the gas test (to see if there is any exhaust gas in the water) so we can't be 100% certain yet.
It would cost at least £800 to do the head, so as the engine appears to be running OK, I think I will just keep topping her up and living with it.