Definatly not the relay. With the Ign off & the relay out and the accumulator pressure released the pedal is still rock hard with no travel until I open the rear brake bleed nipple on one side.
As far as I can work out it is something to do with the block on the side of the M.Cyl. I dont have a schematic of the hydraulic system, but my guess was that the ABS unit dumpped the pressure back to the resevoir until te pedal was depressed and then the abs return valve shut.
But as I say it was only a guess.
Having torn out what little hair I have left I am solving the problem by replacing the whole master cylinder/pump assy with the master cylinder from a Tesla. That uses an electric motor to boost the pedal effort (Bosch iBoost).
Thanks for the reply anyway.