See below for a cut & paste from the boiler manual, which may give you a few pointers:-
31 FILLING
Central Heating
1. Remove the sealing panel. Refer to Frames 44 & 45.
2. Swing the control box down into the servicing position.
Refer to Frame 46.
3. Ensure that the CH isolating valves are open.
4. Fill and vent the system. Check for water soundness.
The safety valve is located at the bottom RHS of the boiler.
The discharge pipe should be positioned so that the discharge of water or steam cannot create a hazard to the occupants
of the premises or damage the electrical components and wiring.
30 SAFETY VALVE DRAIN
IMPORTANT - when filling:
a. Remove the diverter valve actuator (refer to Frame 62)
and depress the spindle several times during filling.
Replace diverter valve actuator.
b. When filling, there may be a slight water leak from the air
vent therefore electrical connections should be
protected.
c. Bleed any air from the pump and ensure that it is free to
rotate



