Explain a rule change to clients

I work in marketing at an accountancy firm. Help me explain a rule or industry change to our clients in plain English.

Context: A Budget or a new HMRC requirement reaches clients through headlines first. They want to know whether it touches them, from when, and whether they need to do anything before the year end.

Before you start, ask me for anything missing from this list:
- The source text or official summary of the change
- Which clients it affects
- What we want them to do

Then:
1. Say what is changing and when, using only the source I gave you.
2. Say who is affected and who is not.
3. Say what the reader should do now, and what can wait.
4. Keep general information separate from advice.

Give me:
- A headline
- An explainer of 300 to 450 words
- A three-line version for email or social

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:
- Every figure and date against the official source, read by a tax specialist
- Whether the change is in force, announced or still under consultation
- A qualified person's review of the 'what to do now' section