I have a Ravenheat Whiter Star LS 80 boiler and all bar one of my
radiators have TRVs fitted. I have recently purchased a Horstmann 099A
programmable room thermostat to fit to the system to give me more
control but I'm having trouble understanding how to wire it.

The diagram on the Horstman RT has three terminals COM, NC and NO. The
diagram is laid out as follows: Live - COM - NO (it says in most
cases NO but can be NC?) - boiler, Neutral - boiler. The boiler
instructions are pretty pathetic, and just show 2 wires coming out of a
terminal block and going off to a room stat. No live or neutral marked
on the diagram?

I'd be grateful if anyone could tell me how to connect this up. Also,
what gauge of wire is best to use.

Thanks in anticipation