how do i get my central heating working again?

Hi,
I have a Vaillant ecotec plus 624 sealed fan assisted boiler.
Just the other day it stopped working and now flashes up 0.2 bar, f22. I've looked in the owners manuel and this fault is dry fire. Too little water in appliance, water pressure sensor defective, cable to pump or water sensor defective, pump blocked or defective pump output too low.

I've tried bleeding all radiators and have opened both the c.h. flow and c.h. return valves under the boiler but have had no joy.
Please has anyone any further advice?
Thanks.