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It actually works a bit differently but there are also some designs of plug that work better with different substrates.
I use different Fischer plugs but always use a drill 1/2mm smaller than recommended. They have one that's better suited to brick, especially where you might be in the mortar.
I will sometimes use an oversized screw to expand the plug that bit more. A 6 plug with an 8 screw happens quite a bit. The larger the plug the larger the surface area of grip between wall and plug so less likely to fail in my experience.
I've hung radiators off plugs of wood smashed into crumbling brickwork, sometimes that's what it takes.
Airhead-
Cheers. Yeah I think it's the Fischer basic ones I've used with some success before, and know the 0.5mm smaller trick from bitter experience. It's not entirely obvious which is the right one to choose in their range but these seem a little more advanced than their basic.
Maybe as per my previous train of thought an 8mm plug with an M6 screw, maybe coach screw if it will work, will serve me best.
Edit: happened to have an M6 coach screw lying around, should work out pretty nicely.
Polygon
Brick, I meant to say. What you say makes sense though, I guess the biggest potential for failure is the interface between plug and wall, so best to use the smallest possible for tightest fit.