Drift is like a spike with a handle which you can push into the end of the pipe. You can remove any dings and make the pipe circular again to get fittings on.
Another cause for sooting up which I did not mention before as I originally considered it to be an easy one to spot but apparently not so, is for some of the early balanced flue boilers the rockwool stuffing can degrade and allow combustion gases back in. To fix you simply need to unclip the front retaining cover and re-stuff with rockwool (wear mask and gloves as its nasty stuff)