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?