My thermostat is fine. Works perfectly, absolutely nothing wrong with it at all. My hot water tank is also fine, again - nothing wrong with it at all.

The pump is screwed. And the pump is under the floorboards.

The guy couldn't fix it today but recommended I put a call to the helpline and they'd send a local guy around who knows the systems better than he does. Oh, and my boiler is now officially 'at risk' because it's an open flue one and there isn't a proper wall vent for it, so I can either cut a six inch hole in my wall or get a new combi boiler...

I'm just full of the joys of spring today aren't I?