I'm looking for clues on where to look with a non-working central heating
system...

The house has:

Oil-fired burner/boiler
Hot water tank
Control box for hot water/ central heating - hot water is working fine,
boiler fires up etc etc, central heating has been turned off over summer
Single room thermostat - set on high...
TRVs on each radiator - set on high...
Have bled all radiators

It *feels* like there's no signal to to boiler to say give me heat - (tho,
as I say, works for hot water)

Should there be a 3-way control valve (over and above the TRVs) somewhere
that I should be able to see?

Help...

Graham