What is this part of my wood burning stove called?

At the back of my wood burner the flue goes out parallel with the floor then turns 90 degrees and off up the chimney. At the 90 degree bend there is an access plug which the chimney sweep undoes to push his brushes up. This plug is damaged (burned through) and the sweep has told us to replace it. What is it called? I have searched online using every name I can think of but no luck. Anyone know what it is?