So far:

1) New Master Cylinder - same problem
2) Changed ABS pump. No change.
3) Capped the Master Cylinder with 10mm modified bleed nipples/screws. Pedal hard and no sinking.
4) Replaced rear circuit (front on Master Cylinder) and pedal hard and doesn't sink.
5) Replaced side bleed screw with second union. Pedal now sinks as before.
6) Checked front calipers. Pushed pistons back into body on both sides. Moved freely using a tool. Sinking returned straight away after pumping the pads in contact with the disc.

I wondered if the issue could be flexible brake hoses but given the front caliper cylinders move back I don't think so.

Fluid level does not drop over time.

Still wondering the Master cylinder but two with still the same symptoms.

Any suggestions gratefully received.