Thanks, guys, for your responses. I have an update...
Much to my surprise, the heating sprang into life at 05:30 this morning (per the timer). It's set to stay on until tonight, but by 08:30 it has cut out again. So it's perfectly capable of working, but it just doesn't seem to want to...
BACKGROUND
Hi, I have this last week had someone in to install a thermostat on my hot water cylinder (it didn't previously have one) and a 3 port valve (I don't pretend to understand what this is for, but the problem it was aimed at fixing was the virtually constant overflow of very hot water...