Write a value proposition

I work in marketing at an accountancy firm. Help me write a value proposition for one service.

Context: Owners cannot judge technical quality in advance. They judge whether the fee is predictable, whether calls are returned and whether someone tells them about a problem before the deadline.

Before you start, ask me for anything missing from this list:
- The service
- Who buys it and why
- What they would do without us

Then:
1. State the problem in the client's words.
2. State what changes after working with us, using proof I supplied.
3. Write three versions at different lengths.

Give me:
- One sentence
- One paragraph
- Three proof points

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:
- Promises about response times or deadlines we can keep
- Tax saving and 'pays for itself' claims
- Fee statements against the price list and engagement terms