Ok, this might sound a rather basic question, but its based upon years
of experience of inadequate wall fixings...

I'm about to mount two large computer desks on battens in our toy room
(cf. "Home Office"), with much of the weight to be taken by desk legs.

The desks will occupy the complete length of an outside wall of the room
(3.6m), and one side is also an outside wall, the other is a stud wall.
These two ends of the table will need battens approx 1m in length
(excessive length due to the curved ends of the desks).

Now, the interior walls are plasterboard over timber frame, so I'm just
going to find where the studs are and screw the battens into those -
that much is straight-forward.

Where I've come unstuck in the past, however, is with the exterior
walls. These are dry-lined, plasterboard blob-bonded to cement block
(with an exterior brick wall - but I'm hoping not to go quite that
far!). B&Q et all sell various devices for various wall types, e.g.
plasterboard fixings, concrete block fixings, etc.

Now, I think I'm right in saying that plasterboard fixings are not
appropriate here - I need the fixing to go into the blocks behing the
plasterboard. BUT, the block fixings that I've seen are intended for
direct mounting (i.e. the head of the fixing is flush with the block -
I've got the small issue of 12mm of plasterboard + air gap in the way.

In a case like this (where I don't want to rip the plasterboard off the
wall unintentionally) any suggestions as to the "best" wall fixings?
(plus, if possible, a URL so I know what you're referring to)

Mike