Thanks for that advise, I've just been out and brought some wall anchors but when I tried to use them they have 2 small teeth that have to be pushed into the plasterboard before you tighten the screw but I found as I tightened the screw and it got tighter as it was opening up inside the 2 teeth started to move and rip the plasterboard and then whole thing started to spin in the hole so I could no longer tighten it up.
Any advise on how to solve this.
Best way to install them is to make hole just big enough for plug end to pass through, then push gripping pins gently in to board. Tighten screw up so anchor expands behind wall, don't over screw when it's starts to become a hard to screw it's there. If that fails and you are having problems. I'd consider trying to fix wooden batten on wall, again with a few wall plugs an some adhesive so you can stick to wall and secure it with screwing tight, leave that to set, hook in to wood. Wood should be of a size, that mirror will hide when hooked up.