Ideal mini c24. intermittent CH, main green flashes normally?

Hi
The Ideal mini C24 boiler is about 4-5 years old. Its been working intermittently for the last couple of years, with the central heating not always coming on when its supposed to. Have even set it to extremes with the radiators all open, the timer (Invensys Drayton SM1) set to 24hr and the thermostat set to 30C !! Nada.

There is also an issue with the system needing repressurising reasonably often. I set it to 1.5 bar twice this week already. This may be unrelated.

Flicked dip switch 4 on the main pcb on and off with no result. There should be a flash code that comes up according to the manual, on the main LED and the red and green 'hidden' LEDs. PCB says everything OK: main green LED flashes normally-1 sec on, 1 sec off.

The main green LED occaisonally does the 4 flash per second thing, but this is intermittent.

Called Ideal, and they said replace PCB. Best part of £140!!

Any Ideas?
Grateful for any help.