Thanks, oilman.
I've given it another, much more thorough clean, and it is running OK again.
(The first time I cleaned it I was a bit cursory). Don't understand how it got so clogged up after less than 5 months, though. We'll see how it goes now.
My boiler suddenly started producing a lot of black smoke about a month ago, coupled with noisy firing. It had not been touched and had been running beautifully since it was installed last September. It was very sooted up inside, so I cleaned it out and that seemed to cure it. Last week it...