I am about to upgrade my existing flexi flue liner (single skin gas) to a double skinned liner for my multi-fuel stove. I have a connector to go from the flexi liner (6") into the rigid pipe (5") and the manual tells me to put fire cement around the joints. There is no fixing other than sliding the 2 pieces together and the cement. I was wondering whether to screw some self tappers in to make it more secure (sealed with cement)? Has anyone done this, any advice?