Having worked as a teacher for over 15 years, 6+ in SEN education reading these threads makes me painfully aware of how hard it must be to get your child the best help if you are not in education and do not understand how it all works. Because of this I have decided to summarise the main points:
1. Diagnosis
Get a DX as soon as you suspect there is something wrong with your child. Push for tests and assessments. You are the parent and you know your child best. If you feel there is something wrong, probably there is something wrong. It won't go away if you ignore it. But valuable time will pass without specialist help. The sooner you get specialist help the less the damage. Don't get fended off by things like "autism cannot be diagnosed this early". It can be diagnosed from 18 months on. Specific learning difficulties (like dyslexia) can be diagnosed before school-age (reading is just the manifestation of a problem). A diagnosis can only be made by a health professional. Someone who is registered with the HPC. A doctor or a psychologist. It cannot be made by the school, not the SENCO and not even the EdPsych (even though the EdPsych will be registered with the HPC). If assessment is arranged via the LA or the GP, sometimes you have to wait months. If you can, go private. If you cannot afford it, look for funding (e.g. a charity). The most important thing is to get a dx and to get it as early as possible.
2. Statement
A statement is given by a panel, which is a multidisciplinary board. It will have an EdPsych, the SENCO and other professionals. LAs and schools are trying to decrease the number of statements, so they will try to tell you that no statement is needed. This is because until there is no statement, all help is essentially a goodwill gesture and it can be withdrawn at any time. Once there is a statement, help for your child is a commitment. It is you who has to push for a statement. The LA and the school does not want statements. Statementing is a long process and you need a dx for it. This is why it is important to get a dx as soon as possible.
3.SENCOs
Schools have to have a SENCO. A SENCO is a teacher (sometimes a TA) who is responsible for SEN kids in the school. Some of them are dedicated and well trained, but no SENCO is a specialist in all disabilities that can be present in a school (from sensory impairment through learning difficulties to ASD and ADHD), so don't expect the SENCO to be a specialist in the disability your child has. Also because having a SENCO is a legal obligation, the role is sometimes assigned randomly to teachers who have neither inclinations nor training for it and who do not get extra time off teaching duties for it. This is especially true in MS primaries.
4. Special Schools
More disabled children thrive in a special school than in mainstream. It depends a lot on the disability, of course, but specialist help early on is gold dust. Do not put your disabled child into a mainstream school just because you, as a parent have the right and because this way the disability can be "pretended away". I understand this may be difficult but again: early specialist help and the right environment is more valuable than being able to say that your child goes to Cherry Tree Primary School, just like everyone else.
5. Don't take "NO" for an answer
"No" means less work for schools, LAs, etc. But it is your child who needs help -push for it.