To me it could be a slow opening contact, this causing arcing and thus radio interference which being picked up by your router wiring.

Try listening to a MW radio, tuned between stations, close to the boiler, programmer etc in turn.

This is similar to the interference you can get from flashing Christmas Tree lights, or a failing fluorescent light tube.

Cheers!

Yup, I'm going to give that a try thanks.

I guess the question is, assuming that is the case - how do I resolve it? I can't just move the boiler and router further apart as they are both fixed points. Is it a case of fixing/moving my master socket to then avoid the interference?

We need to confirm the source.

How old is your boiler, pump and cylinder stat? Do you have a pump within your boiler or is it separate and located say in a airing cupboard?

It has been narrowed down to the boiler turning off, but in my case, the turn off has several steps.

1) The cylinder stat says "I'm hot enough" - and this signals the boiler to extinguish the flame.
2) The boiler keeps power to the pump for approximately 1 minute to allow it to push unheated water through the system to cool the heat exchanger.
3) The boiler turns the pump off

Does your boiler/system have overrun? If so, there are distinct events that occur at different times - the cylinder stat saying enough - and the boiler turning off the pump.

If it has overrun then does the line disconnect when the stat says "enough", or the pump turning off?

If it doesn't have overrun, then what happens if you set a water schedule of 15 mins? By setting the schedule to 15 mins, the cylinder may still be calling for heat when the timer knocks the boiler off. This takes the cylinder stat out of the equation because it is not the trigger for the boiler to be turned off. Apologies if you have already tried this.