Thanks for the quick responses, folks! I realised as soon as I posted
the message that I left out the details, but I hadn't expected anyone
to get back so quickly!

It's a gravity system with a pump. After posting the message I found
the FAQ for plumbing and it described how the thermostat should go in
series with the programmer on the way to the pump for CH. The HW should
go straight to the boiler.

The new programmer is a Drayton Tempus 7 and we have (had?) a Drayton
Digistat 1 thermostat. It's an oil-based system. The programmer is in
the hall and the thermostat is in the living room.

The boiler is a Camray Oil Fired Pressure Jet Boiler 51/67. It has a
Riello 40 oil pump. The water pump is a Grundfos Selectric UPS 15-50.

I don't know what to look for regarding the valve Harry suggested - as
a first time buyer this is the first time I've ever had to think about
a central heating setup beyond programming a timer! I can't see
anything obvious, at least.

I'll check out the link, Mike - thank you!