Thank for the comments all, the wall is dry lined or dot & dab as it is sometimes called which is a combination of block work, and plaster board with an air gap of around half an inch between.
The local DIY store only had wall plugs or drywall anchors, neither of which I trusted enough on their own, but an internet search found something called a DryLine Pro fixing which appears to be a combination of a wall plug and a drywall fixing in one piece, so I gave those a go.
They seem pretty solid and needless to say I put some serious screws in there, which double up nicely as coat hooks on the other side of the wall :biggrin: