I'm open to suggestions on this, i've got a mirror, probably about
25lbs in weight, and need to mount it above the fireplace.

This house is only about 20 years old, so i'm wary that the in-wall
chimney will be quite shallow centrally, so might be a bit of an issue
trying to mount it via a single fixing and very strong wire :-}

The mirror has two large triangular hooks / fixing points already
mounted, but i'm also wary that they won't allow any "tweaking" to get
it straight if either of the holes happen to a bit off...

What sort of fixing method would you suggest please ?

....and if I use the existing fixing points, would you suggest normal
countersunk screws and dangle the hooks over them, or what ?

I did use a central fixing with something like a 3" no.12 screw on a
picture of a similar weight in my old house which worked well, but
again, this is on a "modern" flush chimney breast so perhaps not quite
so advisable...