Try our new info resource - "Aladdin's Cave" (Main menu)Just added a separate link to Ash's Dropbox thread (shortcut)
What a pain Roy, but hopefully you're closer to eliminating all the things until you get on top of it.As Hairy says, it's worth running a bit more to evaluate the pipe and if it will clean up . Although that black plug is worrying and as you say, suggests you've got too much of something burning in there.What about getting a emissions test for each pipe once it's warmed up at mot test station. If you can get a decent readout for each then at least you can make a more fundamental judgment as to what's coming out of that cylinder versus the others.Or, pull the carbs and switch the main jet from that #4 over to another cylinder to see if the problem moves?