I can't really claim any credit but we had a new boiler and HW tank fitted maybe 18 months ago (by an independent highly recommended installer) but just occasionally, the CH comes on by itself !!!!
Son is a qualified gas engineer and it has him stumped, partly because he's often not been here when it's happened...and when he has, he hasn't had the right tools to diagnose.
That said, the firing sequence is understood so it kinda nails it down. He suspects some stray voltage from the motorised valve controls, he's seen it before at work but never with Veissman boilers. Something like because 3 ports aren't spring return, it can cause problems when they get stuck.
So, 'we' converted the 3 port system to a seperate valve/motor each for HW and CH, installed a new power centre (basically the electrical hub/ connection box, just in case there was something in there. Also gave a fresh dose of anti inhibitor for good measure, even though the water in the system came out really clean.
Sit back now and hope that's cured a very intermittent annoying problem.