Attaching a heavy item to a 'powdery' external wall
Here's what I did (probably a little over-engineered but it worked well)


Filled current holes and the surrounding mess
Got 135mm long, 7mm screws with wall plugs, and drilled 10mm holes for them
Located the holes 50mm above the current holes, 50mm being approximately half a course of bricks


Whilst drilling I think the problem might have been that I'd gone between two brick courses, but this was probably compounded by the fact the plaster was thicker than I thought (it might have been 60-70mm before it hit the brick by the noise of the drill), so the ridiculously long far too heavy duty screws probably did the trick.

Each of the screws is apparently rated to hold 90kg, though I doubt either the wall or the coat-hook thing itself would be anywhere near that.