Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Turn Hot Water Tap On
Remove the connector J2 from the PCB and make safe
Replace the fuse (correctly rated)
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Isolate Mains Power to appliance as per TOPs P4

Remove the fuse from its holder and measure the resistance.
Has the fuse blown again?