Hi, our combi is up in the attic. We use what I think is a room thermostat to turn the heating on and off without getting the ladder out, but would like to use a timer - can I simply remove the existing thermostat (more or less the same as this: Honeywell T6360B Room Thermostat | Thermostats | Screwfix.com) and replace it with a programmable timer using the wire already in situ? Thank you in advance...

Most likely yes. I take it that you have heat only (no air conditioning). It would help if you can tale off the existing thermostat and tell us what color wires are connected to it.

I'll do that tomorrow afternoon, thanks :)