I'll try to get in all the needful... We have a newish (not quite 2 years old) combi boiler. Four radiators upstairs, including the one in the bathroom, near the boiler, which is permanently on. There are three radiators downstairs. About 8 months ago, when we had the boiler serviced, we had a fourth radiator downstairs removed, and another replaced with a cast iron radiator, all done by our usual plumber. Pretty much since then, the heating's been switched off, hot water running as normal.
Since we put the heating back on a couple of weeks ago, three of the upstairs radiators are just not working at all. We've bled them - it's not that. The pipes at the bottom aren't even warming up. The rads downstairs, and the permanently-on radiator in the bathroom are all working fine.
Any ideas? It's not desperate yet, but I don't think we can put it off until the boiler service is due 