can you look at the likely plaster depth via a mains wall-box say if it is not discernible via the drill hole
(or evidently by the changing resistance whilst you originally drilled)
I'm pretty it was about 1/2 cm, it's only a skim. Obviously all four plugs are in now so I can't see them, but I believe they had a fair cm or so of 'flat' at the errr.. user end. iI other words the bit that expands as you screw is further down the plug. So that's good.
Girlfriend pointed out that the plumber who fitted our radiators only had his plugs flush (flat to the wall) as we saw them when the decorating was done. IE they weren't pushed really far through plaster into brick or anything. And those rads were damn heavy.
Enough speculation, I just need a break in meetings this afternoon to get the mount on the wall, then perhaps I'll just hang 20kg of weights on it overnight just to be sure, lol..