"I think either corrosion and pad deposits have reduced the effectiveness of the pad / disc"
That's what I think, as a stated much earlier, but I put it down to wear of disc/pad.
There's an experiment that could be done to prove/disprove the theory of "friction has nothing to do with area"
There's a few things that would be needed.
1, need a test car (not mine thanks)
2, two sets of pads, one normal, one with 50% of brake lining removed
3, long road and brick wall
4, someone to drive, I'll put my hand up and test the pads with 100% of brake linings, anyone fancy testing the 50% ones?