Right so I might be making some progress, got a full set of Brembo pads for all corners and swapped them out today.

The two pistons on the inside of both of the front calipers were INCREDIBLY hard to push back into place. I had to pry them back in using the old brake pads with a massive screwdriver to get as much leverage as I could. This is in comparison to the outer two pistons on both sides of the car which just pushed in like a breeze.

Same went for the rear caliper pistons, no problems.
The inner pads were more worn than the outer ones so the pistons were no doubt sticking.











New pads are in, the car is still on stands but after bleeding the pedal does feel a bit better already, albeit not perfect.
I'm going to stick the wheels back on and take it for a spin and see what's what.

The question is, can sticking pistons give a soft brake pedal? The ABS unit is still making that squelching noise so I still think something is amiss there.