I have an old Thorn Apollo boiler, a Randall 4033 timer and a 3 wire
thermostat. Until now this system has been very reliable but now the
radiators now only heat up when the boiler is heating the hot water tank.

The live connection on the room thermostat is live and the switched
connection becomes live when I turn the thermostat up but neither the
boiler nor circulating pump come on. The boiler is in my garage and I
have a frost prevention thermostat - if I turn this up the boiler and
circulating pump come on and the radiators heat up but the boiler
doesn't seem to respond to the main thermostat.

Any ideas what's wrong and how to fix it? I would be grateful for any
suggestions.

Thanks,

Gareth.