Wrighty, 316 grade liner if just burning wood, 904 for smokeless, 150 mm minimum if not DEFRA approved stove, make sure it matches stove choice as some bigger stoves have 170-180mm outlets. If it was my job i would use chim wrap to insulate liner, if your going through at 45* into back of stove, still use reg plate, 1.5mm steel, sweep access if you can’t sweep through stove.
If you can put reg plate above and go through it from top of stove this is the best option, if you want to push stove forward come out the back using a booted T.
Difficult to give best advise without seeing job though.