Update, if anyone is still following this.
Chap came back to fit new filter and hook it all up and now boiler has been up and running for about 48 hours. Crucially the plume from the vent smells cleaner than it has for years.
I really don't want to tempt fate by saying it is all solved, because it is too early to say, but what I can say with some degree of confidence is that it was the clogged in line filter that seems likely to have been the root cause of the problems.
When the chap who reconnected the burner looked at the filter he said that he didn't think it had ever been changed, not even when the new boiler was put in seven years ago (it was a straight boiler swap).
I have noticed this morning that I have a slight oil leak around the isolation valve and filter housing, so I may get an air lock at some stage. However from Monday on the temperature is going to be tolerable enough to have a proper look.
I think all this has really reinforced my existing attitude that you cannot and should not rely completely on other people to do things for you unless you have real cause so to do.
Before this episode the whole system was a series of black boxes to me, and now I have a rough understanding of what the various components do and how they link together. Had I been armed with this information at the start I would have paid attention to the inline filter and checked it after the repairs failed to get the boiler working.
In fairness to the chap who did the final hook up he said he had not checked the filter because he had assumed the other people who had been out had done so, and that links it back to the people who did the service back in December.
All the advertising tells you you mustn't go near these appliances and you must get an accredited person to do this or to do that, but what are you supposed to do when people who advertise themselves as experts are - to put it as kindly as possible - less expert than you might hope?
Don't tell me that you should rely on lists of accredited people, because the people who made the original boiler swap and then serviced it for a few years afterwards were accredited. I have no doubt some or even most of the people on these lists are fully competent, but that is no use to you when you are looking through those lists.
A poster asked if I could not do any of the work myself. Yes, probably, is the answer, but I chose not to out of respect for the welfare of neighbours, and also because the temperature outside was so low I would have been in too much of a hurry. I don't have an analyser or a pressure gauge, so setting the burner up would have been problematical at best, unsafe at worst.
That said, I could smell the plume was not right just by standing in it and inhaling...and this was after the analyser reading had found exhaust fumes to be well within tolerance. Now it smells fine, so maybe your sense of smell is good enough to tell you when al is well.
Anyway, the basics seem to be that the filter was clogged and full of crap from years and years worth of filtering. It reached a point when it was no longer able to filter out impurities from the oil. Those impurities were then being pulled through the pump until they reached the nozzle filter, which was then forced to do the job of the in line filter. As the deposits accumulated on the nozzle filter the atomisation process was compromised, which led to incomplete combustion, which led to the increased smell, and eventually burner lockout.
What I do not know is whether the green film on the filter nozzle was crap in the inline filter being sucked through and deposited on the nozzle filter, or whether particulate dropout in the oil was occurring at the nozzle because the crystallisation process had been effected by the freezing temperatures and could not be blocked by a saturated in line filter.
I suppose it would need a chemical analysis to determine which is which. I would say that out of the numerous samples I took the only one that went cloudy and stayed cloudy was the one taken after the oil had passed through the inline filter. The ones from the storage tank and oil drum were clear and stayed clear even when left out in sub zero temperatures.
