I have plasterboard wall with bit of a gap between it and then a solid wall. Any suggestions whats the best wall fixing to use that will keep the mirror up without falling?
I've had a look at Screwfix would these be ok Wall fixing?

I'd recommend snap toggles, since you have a gap to set them in. They essentially let you have a metal bar with a threaded bolthole in the cavity. I have 8 of these supporting a 50kg Plasma TV hanging off 1.5" thick plasterboard with no issues.

The gap is approx 2cm so the snap toggle probly wont fit in.

Taking a proper look at those Fischers, I reckon you should be alright with them - they seem to be able to behave like other types of anchors depending on the thickness of the plasterboard.

Plasterboard is about 2.5cm thick.

Plasterboard is generally 12.5mm or 9.5mm unless you special insulation board, As a joiner I always advise filling the void between board and wall with expanding foam drill 7mm / 8mm hole in board and pump in.

The foam stops any compression of the board and stops any fixture dragging down always ensure the raw plug or fix is in the block / brick work.

If possible you could add couple of extra brackets to the mirror if they will look symetrical.