what is the correct cable to use to connect a thermostat to a combi
boiler?

A modern boiler and electronic thermostat need just a 2 wire connection. I
normally use 1mm T&E lighting cable, with red being the input from the
programmer (or permanent live) and the black (with red sleeving) being the
switched live back to the boiler.

Older thermostats needed a neutral to operate, too, so needed 3 core cable.

Christian.