Here's a link to the fault finding codes for Glow worm flexi boilers:-
http://www.ribaproductselector.com/Docs/0/22860/external/COL622860.pdf?ac=
F9 Pressure
Water pressure Connector loose,
unconnected, faulty or wires trapped.
Water Pressure Sensor faulty.
Pump ceased, faulty.
No water in the system.
Melanie
March 2008