Even one of those monitor shelves on a flexible arm is only fixed with
red plugs and 5mm screws and seems perfectly sound even with a
heavyish monitor on it.

I use 5mm or 6mm screws into brown plugs myself, drilled with a 6.5mm SDS
bit. The yellow plugs scare me for anothing bigger than holding up a paper
calendar. If I'm doing anything heavy, like kitchen cupboards, then I go for
the full rawlbolt.

Why do you go for the smaller rawlplugs? It's not like screws or plugs are
expensive.

Christian.