Wall Hung External Grant Boiler

[A Nonny Mouse]

Had my boiler serviced at the end of November. Have been having problems with the heating ever since.
Started with the boiler tripping the house electrics when it came on in the morning. Heating engineer sent along an electrician who diagnosed and replaced a faulty valve.
Electrics tripped again 2 days later when the heating came on. Engineer sent second electrician who worked through all the controls, found nothing wrong, diagnosed an unspecified intermittent fault, said he'd need to come back and search for it methodically. He asked us to make a note of details relating to any problems we had. Electrics stopped tripping about 2 days after that.
Then just before Christmas, the boiler locked out, I couldn't get it to restart and stay running and noticed oil leaking from the connector where the flexible hose meets the pump. I drained about 1/3 of an ice cream tub from the base of the boiler. I suspect a fair bit more has leaked over the last few weeks.
Original engineer wasn't responding to calls or texts, so I found a list of local OFTEC registered people, recognised a name and called out someone else who had done some other work for us in the past.

This second guy has made 4 separate visits over the Christmas / New year period. In addition to the leaking oil, he says it looks as though the condensate has been dripping into the workings of the system.

So far, he's replaced the flexible hose and tried changing the capacitor, the pump (including solenoid), the photocell and the control unit. We have oil and it's the right kind of oil.
He has the burner running fine - as long as it's away from the boiler and there is a good air supply. When back in the box and connected to the combustion chamber, it's a very sooty burn and it quickly locks out. The fan is running.

At this point he thinks the most cost effective option is probably just to replace the whole burner. I'm sure he's honest, I can't fault his work ethic, his previous (non-heating) work, or his willingness to turn up and try to fix the problem. I'm happy to pay for a new burner if it fixes the boiler. My problem is that I don't think he has much experience with this make/model and I don't want to pay a load, order a burner, wait a few days more for delivery and fitting and then find I still have a boiler which doesn't work.

Can anyone suggest anything else that it's worth checking, any other things we should be looking at first?

Thanks in advance for any advice.

[A Nonny Mouse]

Please ignore post. Problem now resolved without buying new burner.