Isolate Mains Power to appliance as per TOPs P4
DHW service valve Off
DHW discharged and drained
Flow and return valves isolated and boiler discharged
Note:
Although appliance may be discharged and drained some water will still be present as it is dismantled

Observation:

Remove three-way valve head (held by two clips).
Inspect the diaphragm for splits, damage or signs of stretching.