Merryterry wrote:
I am thinking of putting another handrail up my staircase, (we are
getting old and crotchity),

The lower half of the staircase is ok to fix on as it is on blockwork
but the top section, (1st Floor level) is a studded foil backed
plasterboard wall. I would not trust hollow fixings in the
plasterboard for such a potential load but I cannot identify the studs
because the board is foil backed and my stud detector just bleeps
everywhere.

Any ideas other than making trial holes everywhere?

Obtain some strong thin magnets, some 20mm wide, with the poles on the ends.

Hold lightly in the hand, so they are free to move, but held 1mm above
the face of the plasterboard.

Then 'scribble' over the surface of the plasterboard, the magnet bangs
against the surface when you are over a nail.

http://www.mauve.plus.com/walls/dsc01458.jpg shows after I did this all
over the wall, cut the plasterboard top and bottom, and dremeled out
circles round the nails (quite fast actually, and the sheets are
suitbale for re-use doing the garage).

However, in your case, once you find several nails, you can draw a line
and find the probable centres of the studs.