Alex wrote:
That's what I did in mine but I used stainless steel folded 25mm all
round then predrilled the holes in the fold down and used normal
plugs with stainless screws,also spread fire cement on fold before
fitting and it gives a more secure seal.the folded area was slightly
sloped out on all sides.It is much harder to work with but as I have
no liner in my chimney I felt it was more secure.As you have liner
mild steel would be more than adequate.

Alex

If I may ask,how much did you pay for the mild steel?
Last job I done the guy paid 18GBP for a sheet of cement board.
Bitch of a job doing it that way.

--
Sir Benjamin Middlethwaite