Brief an agency or freelancer

I work in marketing at an accountancy firm. Help me write a brief for an agency or freelancer.

Context: Agencies new to accountancy reach for 'save tax' headlines and stock deadline dates. They underestimate how often figures change and how long a partner takes to approve.

Before you start, ask me for anything missing from this list:
- What we want done and why
- Budget and deadline
- Who approves the work and how long that takes

Then:
1. State the outcome and how we will measure it.
2. List what they must and must not say.
3. Explain our approval steps and timings.

Give me:
- A one-page brief
- A list of what to send them with it

Rules:
- Use only the facts I give you. Where something is missing, write [detail needed]. Never invent a number, a name, a quote or a source.
- Give general information only. Never write anything that reads as tax advice on a person's own affairs, and never suggest a tax saving or an outcome with HMRC is likely or guaranteed.
- Write in plain English, in our voice if I have given you a sample.
- Do not tell me what the rules or regulations say. If something may be restricted, flag it and I will check.

Finish with a list headed "Check before this goes out". Include these, plus anything else the partner who approves marketing or our ethics and compliance lead should see:
- Our list of words and claims we do not use
- Who checks tax figures and how many working days they need
- What client data and software screenshots the agency must never be sent