Liners are commonly secured at the top.Yep looks that way now.
I would like to thank you for all your advice. Really cleared some areas up for me where I was struggling.
Now just got to design the supports/clamps. I'd like to have one just between the main 0.5m black stove pipe and where the liner starts. I'll be choosing the rear exit pipe on the stove (have both options).
1 curved 90 degree bend at the rear (has a removable plate for cleaning) and then attached to the thick 2mm black stove pipe, which will just go through my closure plate and then have the liner start there. So will have some extra support just above the closure plate (which I'll make removable).
That's the plan anyway.
Possibly some strong brackets on the wall with a 130+mm jubilee clip or home made clamp from some 23cm galvanized piping I have leftover.
Jubilee clip holding some L shaped supports around the pipe that will rest on a couple of strong shelf brackets attached to the wall. Have that kind of design in my mind.
Do you think you could make something like that work? Otherwise you're looking at rigging up some brackets lagged into the masonry