Tell the engineer which supplies are currently isolated and why. Do not reconnect a disposer that was isolated because of a leak, electrical fault or unsafe behaviour merely to make the visit easier.
What the observation tells you
The engineer can decide how the assessment should proceed and what safe supplies are required. An unavailable water supply, inaccessible isolator or shared connection may affect the planned visit, so disclose those conditions beforehand. Avoid disconnecting other appliances without understanding their role.
A useful next step
Keep the existing safety arrangements in place and provide the relevant controls and access information. Agree any preparation directly rather than performing a trial run before the appointment.
Manuals and supporting guidance
Model-specific instructions take priority. These references support the practical principles discussed here; they do not diagnose your appliance.