There is a significant difference between an informal chase email ("Just checking in on the progress of our assessment request") and a formal complaint. Local authorities know the difference. Informal chasers are easily deprioritised. A formal complaint creates a statutory obligation to respond.
Here is how to write one that actually works.
Start by establishing the breach precisely. Name the date your request was received, the statutory deadline that applies, and the date that deadline was missed. Do not generalise. The more specific your complaint, the harder it is to dismiss.
Use the correct legal references. The Children and Families Act 2014 and the SEND Regulations 2014 are the relevant instruments. Citing section 36(3) (the duty to assess) or the 6-week regulation (Regulation 4) shows you know the law and are not guessing.
Address it to the right person. The complaint should go to the Director of Children's Services or the Head of SEND, not the caseworker. Copy in the SEND team leader and, if you have one, your key worker. BCC your own email address so you have a timestamped record.
State what you want clearly. You want a decision on your assessment request by a specific date. You want a written explanation of the delay. You want confirmation that the 20-week timeline will be reset to a realistic date from now.
Attach evidence. Include your original request letter, any acknowledgment received, and a timeline of contact attempts. This establishes the factual record the complaint needs.
State your next step. End with: "If I do not receive a response by [date], I will escalate this complaint to the Local Government and Social Care Ombudsman and contact my MP." This is not a threat. It is a factual description of your rights. Stating it changes the urgency with which your complaint is processed.