Question Author
alava: that's the key to the problem.
I am disabled, and have to fix various devices to the walls - grab rails, fold-down bars to go next to the toilet, and so on. And they are mostly metal, and they come mostly with pre-drilled holes for screws. So to get a frame fixing screw into them means first drilling out the smallish screw hole with a metal drill to enlarge it enough to take the bigger fixer.
They take a lot of bashing - I'm heavy - so need to be well anchored into the walls, i.e. not tiddly little shelf, but something to take a large person heaving themselves upright!