Revaluation of non-Current Assets - Letter of Engagement Template

Hopefully somebody is able to assist us. We are trying to put together a letter of engagement to our internal valuers of PPE etc, and wondered whether anybody is able to provide us with a template rather than us re-invent the wheel so to speak. The instruction for annual valuations has to date, been informal but our auditors, quite rightly, wish to see something more tangible.

Any assistance would be greatly appreciated.