Our gas fired ducted air central heating has the combustion products exhausting through a duct that passes through the loft space. The duct is about 6 inches in diameter and is made from an " asbestos like" material. The duct has been broken at a junction between two sections. The swagged junction is broken. I have reassembled the duct work effecting a "repair" using gaffer tape. How do I make a more secure repair?