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Unlikely by programmer, likely by the valve.

Most likely the middle position valve is letting by to the heating path.

That part is indeed obsolete. But it is a piece of Al, how can it be damaged? I would suggest you to go for a second opinion.

Was the whole system working last year?

From my memory this is a positive pressure boiler, be carefully on case refitting and seals!

Believe so the OP said heating will be off when hot water is on

hex = heat exchanger. Plate hex = hot water heat exchanger.

Might be a heat only Solo HE / Promax SL

Hot water temperature sensor might be out of range. Or plate hex is blocked.

Just checked there are two solenoids one for pilot another for main? What if the main solenoid failed? Edited, 112 you are right

Afraid no. If the flame distinguished the PCB will withdraw the power to the gas valve, like @picasso said.

Correct. Do you understand what are T1 and T2 for?

No of course you also need L and N and even T1 and T2.

OP did have some confused description about bang sounds which lead us to think it is a circulation problem, but as he said eventually he would get lukewarm rads, so to me that is not for a blocked cold feed. I suspected the APS, the same principle as the solenoid.

Agree with you.

Remove the link between 3 and 4 on the boiler. Boiler 3 to Heatlink 2 Boiler 4 to Heatlink 3

I didn't install any filter on my own house.

Could it also be the APS, play up by thermal effect? I had one two weeks ago, fan was okay but APS intermittently cuts itself off. I put a link at APS to test, the boiler stayed, so replaced the APS, not been called back.

Replace

But forgot to show a hot water cylinder stat between 6 and hot water valve