The flue spigot or flue collar connects the stove to the fluepipe which in turn enters the chimney. Over time, the inside of the flue spigot tends to build up with tar and soot deposits. This does get swept out when the chimney is swept, but tar and soot are corrosive and with the high temperatures that are endured this is a part that often needs replacing. This is quite straightforward. The flue collar will have been sealed with fire cement. Chip away the fire cement to release the flue collar from both the stove and the flue pipe. Clean up all the surfaces and add a thin layer of fire cement to both sides of each part. (Fire cement does not keep well, so do not use if it is has been hanging around in the workshop for years, buy fresh). Replace with the new flue collar. If there is a larger gap it is best to use ceramic rope with the fire cement. Black fire cement is available, as well as the traditional white, which may disguise the seal better. The fire cement should be allowed to dry slowly, so allow the stove to heat up very slowly over a 3-4 hour period, gradually increasing to full temperature after that.