What brake pads/discs are in the car? lower quality brake pads can often be the cause of spongy brake pedals and no amount of bleeding will fix the issue.
They're OEM dealer pads and discs.

Are the brake pad retaining clips still in place? If not these are needed as they apply tension and reduce caliper movement in turn reducing pad movement.
Yes they're all in place and in the correct position.

Have you tried changing your brake pressure switch/accumulator on the back of the ABS/DSC pump?
I haven't changed this no, but there was no code for it, would there be if it was bad?

Have you inspected the brake booster vacuum hose, elbow and gasket to booster for damage etc?
Yes no damage to the booster line. Even if there was, surely this would cause a heavier pedal.

Have you checked for error codes in the ABS system? If so have you cleared codes and plugged back in after a drive?
Yes there were no ABS related codes, anything in the history was cleared and no improvement.

Are you bleeding brakes in a Z pattern, starting with the caliper furthest from the brake reservoir?
Yes indeed I am.

Does the ABS work if you are to brake hard?
Can't say I know this for sure as i've only recently bought the car back and its been up on stands for the last few weeks. But the pump activates and all the valves actuate through INPA.

Has the brake master cylinder ran dry at any point in the bleeding process?
No it has not run dry at any point, either through pressure bleeding or the two man process.

I should have new pads arriving tomorrow as they're low all round, so will inspect all calipers then, thanks for the input.
I'm still baffled.