Isolate Mains Power to appliance as per TOPs P4
DHW service valve Off
DHW discharged and drained
Isolate flow and return valves and discharge boiler

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) to expose the diverter valve body.
Depress the centre of the valve. Does it move freely and return to its rest position?