When I say complete switch off, I mean the timer controls.
They're both showing off, but the pump and boiler keeps going.
The pump gets progressively louder the longer it runs on, which I assume is because no water is going through it (the radiators do cool down)
If I press 'advance' and make the heating controls light up again, the pump goes back to being quiet after a while.
So it would suggest the pump is running on, but some valve or other is closing off water to it.
When I press 'advance' this valve is opening again and allowing water to go through the pump again.