Lobster wrote:

I then use 3.5mm or 4mm screws in the yellow plugs and, as I said,
have had no problems at all with their security.

I really can't imagine what I'd want to fix with big plastic plugs.

I'm guessing you live in a modern house with solid brickwork and maybe
sand-cement render on the internal walls?!

I hardly ever use yellow plugs, simply because they aren't deep enough to
get through the half-inch layer of powdery plaster which was applied about a
century ago, and wouldn't take any sort of weight whatsoever.

Well push them further in then! There's no requirement that the plug
has to be flush with the surface. Or you can put two plugs in if
using really long screws.

My 'standard' screw tends to to a 4 x 40mm, that's pretty similar to
an 8 x 1.5" but a bit longer. If there's half an inch of plaster then
a yellow plug will nestle neatly down below the plaster.

As it happens my house was built in the 1920s.

--
Chris Green