I've been up in the loft again this morning to check the tank. I took the precaution of taking 5 4-pint bottles of water up with me. It turned out there's still lots of water in the tank, but of course the heating has hardly come on because it has been so warm. I'm hoping there is no leak: the day we had the new boiler fitted, in January, there was loads of water in the tank, with a layer of ice on it. There had been no leak at that point. But of course the system was drained so that the boiler could be fitted, and it has had limited use since. Our system is full of potential airlocks and a couple of the upstairs radiators seem to need bleeding, but there doesn't seem to be much pressure from the loft tank. That implies to me that there is either an airlock or a sold blockage somewhere. All the ground floor radiators are full of water.
Today I have bumped the thermostat up to male sure the boiler stays on for longer, and forces the water to circulate. I'll keep on checking the radiators.