If you want your school aged child to have a health assessment related to behavioual issues your GP will not make the appropriate referal unless you have completed a Request for Health Assessment form in conjunction with your child's School.
You should be able to get this form from your child's school, the school nurse, your GP or by downloading a copy from this webpage which can be found in the downloads section on the right-hand side of this webpage.
Ambitious about Autism produce A guide to autism in the early years - Right from the Start that you might find useful.