The other end of the black sheathed wires are deep in the bowels of the boiler! No idea why it wasn't attached but boiler has been working so far (last two years, system was installed when we moved into the property). Plenty of fuses on stand-by!
Rolling up sleeves and installing the Nest thermostat we treated ourselves to for Christmas.
Attached some screenshots of previous and new wiring to check before switching it back on.
Anyone able to cast an eagle eye over these to make sure looks right?
Black wire is boiler and White wire is...