Chris,

Thank you for the courteous replies.

If the head of the screw *isn't* larger than the O/D of the plug then,
to my mind, the plug is *way* too large for the screw. As I keep
repeating I use 3.5 and 4mm screws in yellow plugs which go in 4mm
diameter holes in the wall. Why use anything larger?

In the case of socket boxes I'd say that the thinking behind wanting to use
a larger plug is psychological: the fixing has to hold against what seems to
be a considerable force frequently exerted against it, when plugs are
removed from sockets. What's more, that force is in the exact direction
which could pull the plug from the wall - not like, say, a wall unit or a
picture, where the force is at ninety degrees.

Given all that, those 4mm yellow plugs just seem somewhat flimsy.

Bert
http://www.bertcoules.co.uk