The common assessment framework (CAF) seems to be the way to go nowadays. It brings all agencies together under one umbrella for the good of the child. I would imagine a first stop would be doc or health visitor or school nurse etc.
I'm not sure how well it is working in practice as I haven't been involved in a referral yet, but it can't be worse than the old system - hopefully!
You can find out more here
http://www.everychildmatters.gov.uk/...gservices/caf/