Long nose makes everything easier as the nose slides into the vitreous and provides telescopic adjustment whilst fitting. I HIGHLY RECOMMEND THIS PART.
The top of the adaptor connects to your flue liner (self tapping screws to secure). The bottom of the adaptor connects to your vitreous flue pipe (pipe that leaves your stove) using two metal to metal self tapping screws. You will actually need to drill a small pilot hole for the self tappers (due to the stainless being extremely tough).
When attaching the flu liner to a long nosed adaptor what size self tappers should I use and does the screw go through the liner and the inner wall of the adaptor
5-7mm should be fine. Yes all the way through both. You will likely need to drill a tiny pilot hole as the stainless steel is very tough.