Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Isolate and drain DHW circuit
Remove flow switch from its housing

Observation:

Remove the DHW filter / restrictor and check for any blockages or restrictions.
Note:
It may be necessary to remove the gas valve to gain better access to remove the DHW filter.
If gas valve is removed ensure all joints are checked for gas tightness on reassembly.