Boiler firing up when programmer switched from on to off

Hi,

Hope you can help steer me in the right direction for this fault.

So I have a gravity fed heating system, with a Potterton Profile boiler, Drayton Lifestyle LP722 Programmer and Drayton Combi-stat.
(Also Honeywell 3 port valve).

This has all been working well for a few years, however have had it switched off completely over the summer and when I've switched it back on the following happens:

Press Heating on button alone (on Drayton Programmer) = boiler does not fire up
Press Hot water on button alone = boiler does not fire up
Press both on together = boiler does not fire up
Switch either of them off (when set to on but boiler not firing) = boiler instantly fires up (every time)
Then I am able to switch the other one on and both heating and hot water work perfectly until they are turned off.

Just trying to diagnose if this fault lies with the programmer or boiler?

Hope someone may have seen this fault before and be able to advise?

Many thanks

 

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