Thanks all,
Corgi block came yesterday, removed burner assembly and scrubbed away a ton of crud. Did a lot of work with the hoover on that and inside the boiler casing. Add to this, he added a 900mm extension and bracketting to the vertical flue (all now stands proud 1m above the building), and he made me improve the ventilation (fitting an ouside air brick and partition door vents) so that the boiler has better oxygen at the burners.
He reckons about 10 years at least left in the Potterton Kingfisher beast, and I'm happy with that. Cost of work was about £300 which although steep for my finances and not quite in the DIY spendthrift ethic compares with experts that sort out my neglected car every time it gets the MOT fail certificate. It's a lot less than to start paying for installation of a new boiler at the moment, even bearing in mind the projected efficiency of a replacement.
A yearly service from this bloke would cost about £60, and his contact number is now close at hand - having gone through so many friends for recommendations to find most gas fitters very busy or not checking their voicemail. I should give up software writing and become a fitter myself.
Great to have a decent bath again :-)
-- Adrian C