Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Turn off cold water inlet to boiler
Drain water as appropriate

Observation:

Remove DHW flow switch from boiler. The flow restrictor if fitted will be situated in between plug A and the body B. This can be separated by levering with a screwdriver one of the two hooks.
Clean the filter of any blockages or restrictions. And check it contains the correct flow restrictor for the model.
Flow limiters are available in the following sizes:
Nominal Flow RateColour
10Blue
12 Red
14Pink