Do you keep it on continually?
I had a nightmare with mine when we moved in, it didn't seem to heat up, we turned it off this summer, but I need to think about turning it on now to make sure it works.
I turned all the thermostats back on today and set them for 21 to test it, but the boiler didn't make a noise so I don't know it it worked or not!! The temp has reached 21 and it has clicked off, so it looks like it works, but I'm not sure if it didn't just heat up to that during the day IYSWIM?