Ariston Euro Combi SX20 MFFI - Intermittent Pilot Light Failure?

My boiler pilot light goes off and on intermittently. The fan seems to stay on but the pilot light doesn't. You can sometimes hear the boiler re-igniting several times before it either stays off or stays on and then you can sometimes hear it re-igniting over the operation period. It is intermittent and can fire up with no problems on occasions. Twiddling with the tap or switching it off and on can trigger the pilot back into action. My problem is everyone is off a different opinion what it is. British Gas said possible diverter valve (not sure why) but wouldnt repair under contract cause of age. Independent plumber said PCB although he didn't seem to do any specific test to check this and Ariston have now suggested its the APS after I emailed their technical department. I don't mind getting it repaired but I don't want endless costs till someone gets it correct. Any ideas would be appreciated.