Hi
The best thing you can do is gen up on the SEN system and inform yourself as much as possible. Its a steep learning curve, but there's tonnes of help on the internet and anything you don't understand you can always come on here and ask. 
The DirectGov website is a good source for learning about the basics of the system and how it should work. You can follow the links at the bottom of each page right from early identification of SENs through to the school system and ultimately statementing.
Another thing you should do is get hold of a copy of the SEN Code of Practice (SENCOP) and Disability and Equality Act as these will give you the back-up you need in terms of your and your dc's rights according to law.
I will try and explain it, but am far too wordy and will probably make it sound more complicated than it is. So feel free to ignore me and go straight to the websites.

Diagnosis does not guarantee a statement, but neither does the lack of a diagnosis automatically deny one. Basically there are three levels of SEN support in the educational system.
School Action (SA) is for children with an identified additional need and means the school is aware and in theory, is putting in additional support to help overcome any barriers to learning or achievement. The child should have and Individual Education Plan (IEP) that sets out individual, measurable and achieveable targets for the child and best practice is for this to be drawn up with the involvement of the schools SENCO (Special Educational Needs Coordinator), the teacher and the parents. This should be monitored against achievement of targets regularly - preferably half termly, but more usually termly and a new IEP drawn up each time. There is no legal requirement for schools to carry out the additional support a child needs on SA.
If, either the child has such SENs that additional school support is not going to be enough OR the child has been on School Action with little or no progress and further outside assistance needs to be sought then the they would move onto School Action Plus (SA +). This basically means that outside professionals are involved with the child. This could be a Paediatrician, SALT, OT, Educational Psychologist, SEN or ASD Inclusion Team, CAMHS or any other relevant professional. There is no legal requirement for schools to carry out the additional support a child needs on SA+.
If it is apparent that the child cannot make adequate progress or access learning effectively even with the help at School Action Plus, then either the school or Parent can apply for Statutory Assessment with a view to the Local Education Authority issuing a Statement of Special Educational Needs.
You do not however, have to wait for your child to go through SA and SA+ before applying. As long as there is sufficient evidence, be it from the reports of outside professionals or the school itself to suggest that your child will be unable to effectively access learning and make adequate progress without more support than the school can provide from its own SEN Budget, then you can apply. All you have to do is write a letter to your LEA stating that you feel your child needs more help than the school is able to offer and would like them to undertake a Statutory Assessment of his/her needs. They will then send you a pack with forms to fill in, asking for you to send them as much evidence as you can muster about why you feel your child needs a statement. They will also contact any professionals involved with your child and of course the school. They have 6 weeks from when you first write to them to let you know whether or not they are willing to carry out a Satutory Assessment.
The charity IPSEA is a good place to start and great for helping you through this process. Another good one is SOSSEN!
If they do carry out the Statutory Assessment, they will then do one of three things.
- Agree your child needs a statement and draw one up for you to approve. It is vital that you make sure the statement is both specific and quantified - not just 'X needs additional support with Y' but 'X will have x amount of hours support with X from a suitably qualified person - which will include a, b, c'.
or
- Agree that your child needs additional support, but state that the school is capable of providing it from their own budget and resources. If this is the case they may issue a 'Note in Lieu' stating this fact.
or
- Simply turn you down, possibly stating that the child has sufficient support already.
You can appeal either a decision not to assess or a refusal to statement and there is the SEN and Disability Appeals Process to help you with this - but I won't go into all that now, as hopefully you'll never need it.
A statement isn't always necessary, if the child is well supported at school, happy and progressing and achieving well then in theory it shouldn't be required. The problem with this is that without a statement in place detailing exactly what additional support the child should be getting, then there is no legal requirement for the school to support them. It really depends on the individual school and their attitude and policies to SENs etc.
Hope this helps a bit. Its a lot to wade through I know, but once you get your head round it its not that complicated. I'm afraid my ds wasn't diagnosis early so I can't help with the process coming through portage and early years support - there are lots of other people on here who can though. 