Tim Watts wrote:

It's odd. I made littlw holes with a drill to test. There's
plasterboard, then behind it there is a gap, and only at about 40mm is
there anything (which seems to be brick and mortar).

Daniele

There's a solution to that if you need to pull something tight to the wall.

Mark the fixing holes and drill them through the PB only.

Using a throwaway PU foam can, fill the void with foam behind each hole in
all directions.

When set, drill again, into brick, rawlplug the brick and fix. The foam over
about a 5+cm radius will provide a lot of support to the PB.

Very clever.

I've found a way to move things around - I can have the bottom of the
shelf resting on the skirtingboard, so that will make the fastenings all
a bit less critical.

The fastenings' main (and easier) job will be to keep the shelves tight
to the wall, and not hold up all their weight too.

I think I will try to avoid having to go into the brick, and I'll use
the kind of bolt that expands outwards behind the plasterboard as it is
tightened. Here in South Wales houses of this age all use a filthy kind
of black mortar, which I inevitable drill into, if I don't just hit the
edge of a brick to make the hole go sideways.

Daniele