Timbo wrote:
Unfortunately there is a steel lintel right above the plasterboard
which prevents me top mounting the blind in the way I've done before,
using plasterboard plugs.

Most plasterboard fixings need quite a lot of room behind the board to
enable them to expand. These Fischer Plasterboard Plugs use much less space
(see pictures)
http://www.screwfix.com/app/sfd/cat/...6589&ts=89807#

Or you could try these Metal Easi-Drivers , very strong fix & not much
longer than the thickness of the board
http://www.screwfix.com/app/sfd/cat/...90027&id=11595

With 12mm board & a 3mm skim you should have room for either. Stick a
bradawl through till it touches the lintel & measure the depth.

As the windows are upvc I'm really not sure
about making holes in the window frame, how to do it or whether it
would work. The builder has advised that the window company won't
invalidate the guarantee if I do this.

I have used twin thread woodscrews before, if the client gives permission.
They grip into upvc very well. Straight in with drill driver, no pilot.


--
Dave
The Medway Handyman
www.medwayhandyman.co.uk
01634 717930
07850 597257