Just like to add that I have now managed to access the boiler condenser trap and it was, as suspected, totally jammed with debris which I had to dig out with a screwdriver. The job wasn't as bad as expected provided that you can gain access to the side panels so that they can be loosened enough...
I too have an HE260 which seems to be doing a similar thing.... If I listen outside to the flue pipe or unscrew the small inspection screw on the flue pipe just above the boiler it sounds like a washing machine (very watery). Also when it goes to light it sounds like bubbles being blown through...