Write a proposal cover note

I work in marketing at an accountancy firm. Help me write the note that goes with a proposal or quote.

Context: The note sits on top of a fee proposal and engagement letter that set the scope. Owners read the note and the monthly figure, and assume everything discussed is included.

Before you start, ask me for anything missing from this list:
- The proposal summary
- What they said mattered most
- The next step and any deadline

Then:
1. Open with what they told us they need.
2. Say what we will do and what it costs in two sentences.
3. State the next step.

Give me:
- A cover note under 150 words

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:
- That the note matches the engagement letter on scope and fees
- Assumptions behind the fixed fee, such as transaction volumes or record quality
- What is listed as extra, and at what rate