Baxi Combi Instant 80 he pump fault/low pressure fault?

Hi, I have changed the differential valve dhw on my Baxi boiler today, and fired it all back up and topped up the CH pressure to just over 1 bar. And now can't get it to go, the Pump Fault/low pressure fault light flashes when you turn the pre heat on. Have tried bleeding the pump and all radiators but to no avail. Don't know if this is an easy fix or if I need to call in the experts? Thanks