IPSEA and AFASIC helplines are good with advice on this. If a child has SEN and this is noticed, the school should put them on SEN Support. This means you should meet them 3 times a year and they need to keep SEN records (sometimes called a provision map or an individual education plan). They should tell parents if the child is being put on "SEN Support" - but it certainly wasn't clear to me when this happened to DS 2 years ago! So if you are not sure - ask. Also ask for copies of all the school SEN notes on your child as the records are very handy - our school said DS was "fine" but their own notes highlight stress and anxiety - ie "not fine".
It's best to stay out of discussions about money but in theory a school would normally be expected to put quite costly or complex support in place without an EHCP. In rare cases this could be up to £6k per year but in practice it will be a lot less. In practice there seems to be no accountability about spending, so the school can accept the SEN top up funds from the LA but then not necessarily spend it on SEN. It's all very murky. Anyway the Internet is littered with tales of mainstream kids on SEN Support with little/no/inappropriate assistance. As someone said above, it seems like some schools are good at this and many more are not. It's a lottery.
Many people are told that their kids will not get an EHCP but lots who are told this maybe are eligible! So it's best to work it out for yourself.
Get all your paperwork together and read it carefully. Make a list of all the "needs" listed in the reports - or all the things your child needs support with. Make another list of all the "provisions" - all the therapies, accommodations and interventions suggested.
If after that you think that your child has "complex needs which affect their day to day activities" then consider applying for an EHC yourself - do tell the school first tho!
Use a sample letter off the internet (some of the wording is important to trigger the statutory process) to request an EHC assessment and pad it out with a list of your child's needs/provisions and attach evidence (that is reports or meeting notes, SEN support records). (You may need to go back to the professionals who wrote the reports to get them to issues clarifications - lots of problems are caused by the diagnostic reports being vague or out of date it seems). You probably will need to have evidence of your child having some sort of Specific Learning Difficulty (ie dyslexia, dyscalcula or dysgraphia etc) - which is possible whether they are working below/at/above average age standards. You need to make the case that his needs are complex or severe to be one of the 2-3% who qualify for an EHCP. I'm hoping that "so subtle the school didn't know what to do" = complex but I'm not sure that's going to work. Send to the LA.
Legally if a child "may have SEN" and "might benefit from a plan" then the local authority are obliged to do an EHC assessment. Often they refuse. IPSEA advise families appeal which might involve tribunal. (They have special advice packs if you get into these more complex situations and advise you seek advice.) most appeals go in parents' favour.
After an EHC assessment lots of LAs go on to issue a draft EHCP. Whatever happens at this stage seek advice from IPSEA as they have top tips to save you from pitfalls at every turn!
Anyway hope that answered the original question: the school should be identifying needs and putting evidence based provision in place with targets and timescales. Schools are obliged by law to provide support for SEN children to "achieve the best possible outcomes" and also to use their "best endeavours"...... Vague does not cut it. In reality you will need to do a lot of encouraging to extract decent support.
Sorry about the mammoth post.....good luck