When you write your complaint you should state that you believe the incident was caught on camera and you should ask to see the footage.
State clearly in your complaint where the incident happened, and draw a small diagram of the area and mark where the CCTV cameras are that would have caught the incident. Also state what the date and approximate time was. Make it very clear in your complaint that you want that footage kept, because it’s normally only kept for a limited time. Make your complaint early and quickly, and keep a copy of it for yourself (write it out twice if you have to, so that you have a copy to keep).
If you have a lawyer helping you it may be better for them to ask for the footage.