Well, with a bit more hunting around online and some tinkering, I discovered that the thermostat has a reset button, I guess designed for the very problem that I had! After resetting it, it appears I'm back to normal. Phew.
But it does raise the question of how I can safely fill the boiler without causing problems with the boiler and without causing the reset button to activate again. I have thought of disconnecting the boiler while it fills up and then reconnecting.
Any thoughts? Thanks.