Who should I write to at the LA to warn of a potential JR claim?

For SEN cases, write to your case officer and copy in the Head of Department. For issues that are not limited to SEN, write to the Head of Education. For a transport issue, write to the head of school transport but again copy in the Education Department. You should be able to get addresses from the website; if in doubt, use the central Town Hall address and/or phone for the address.