S-Duval F30E boiler problem?

Saunier Duval F30E combi:
HW is okay on its own. CH is okay on its own. But when CH is on and HW is turned on, after a few seconds boiler locks out with F9 displayed and red light flashing.
Water pressure sensor (S-D recommendation) has been replaced, but problem persists.
Gas stepper motor checked and opens and closes in simulated demands
3 way valve head checked and piston(pin) moves in and out on simulated demands.

Any help will be v highly appreciated.
Thanks