Appliance Repair Questions? Ask a Technician for Answers ASAP
Good morningI don't claim to be a W-B Heatslave expert, but I'm a heating engineer and have worked on oil boilers so rather than leave your question unanswered I'll offer what thoughts I have.With intermittent lockout I would check the basics:1. Fuel starvation;2. Dirty or failing optical sensor;3. Earth terminals loose or corroded.The third of those is the quickest and cleanest task, so I would start with that.The optical sensor is pretty easy to clean and/or swap out, if you haven't done that yet.With the fuel line, I would check and clear any external filter (between tank and boiler), and blow air back all the way from the fire check valve so that it's bubbling into the tank. If necessary, rig up an adapter made from duct tape and use a bicycle pump to blow back - anything will do.
My background is also in electronics, but this isn't a competition of academic qualifications.If there's a heating demand then the boiler will go into the ignition sequence. Lockout results from a big range of possible causes, mostly related to fuel or electrode problems that lead to flame failure (or detection thereof).You're tending to dismiss my recommended checks on the basis that you can eliminate those possible causes by hypothesis; I think that's fallacious. I feel that you want the fault to be in one of the thermostats, but I don't concur with your analysis. If you're wrong about the earthing, then all bets are off, hence that aspect needs to be physically checked. If you're not prepared to check the basics that I would be checking if called out, then I can't help you.
... as indeed I probably would have done if visiting.