Paul wrote:

Can I put two large screws with spring toggles, or similar, into the wall
and just suspend the mirror off those?

I would use the screw-type plasterboard fixings.

http://www.screwfix.com/app/sfd/cat/...91620&id=11923

I have used these to hang radiators on the walls, and the radiator
weighs more when filled with water than any of the mirrors we have (and
that includes a mirror we have which is real plate glass and a heavy oak
frame).