I've got an old 7'x4' mirror wardrobe door which I've decided to recycle
in the refurbed bathroom - basically by removing the glass, cutting it
down a bit and hanging it along the complete length of one wall, above
the basin. (A la Property Ladder!)

The actual glass weighs about 25kg. The wall is a freshly re-skimmed
lath-and-plaster job and is pretty solid; I was thinking of sticking the
mirror to the wall with some form of adhesive (probably standing it on
temporary battens screwed to the wall until it's set). Using screw
fixings would be awkward because (a) I haven't got the bottle to drill
holes! and (b) the positioning of the holes relative to where the
supporting studs are would look crap.

So - never having tried this before, is that appropriate for adhesive?
What to use?... not sure there's any particular need for special mirror
adhesive (unless that's best anyway) in that the back face of the mirror
is protected by nylon-thread reinforced plastic film (for safety, from
when it was a wardrobe door).

Should the plaster be treated first? With what?

Thanks