I have been waiting for a proper assessment for DS2 (now in Yr 7) for the last 2 years! Finally it's happening 
. It seems that there will be lots of different specialists there and I may finally get an opinion which we can use to get him the help he desperately needs.
He has seen a paed a few times now and had a provisional diagnosis of 'learning difficulties' and I was told that he has the mental capacity of a 5 year old (when he was 9). His primary school have been totally useless and although he was on an IEP (school action+) since Year 1, he still finished up Yr 6 working at a Yr 2 level. They insisted that they felt he did not need an assessment as he was just 'lazy' and 'badly behaved', needed further boundaries at home and I needed to make him do his homework (homework = meltdowns = smashing up his bedroom and his twin brother).
He saw an Ed Psych twice in his time there but the school did not carry out her recommendations and until Yr 5, (when I started panicking about secondary school and how DS would cope) the school did not support me at all with any further intervention. My GP would not request an assessment as he said the school should request it which they would not do so nothing happened except for DS2 falling further behind and me being at my wits end with his behaviour!
It probably did not help that his twin brother is a high achiever (top set for everything), very popular and always displayed exemplary behaviour. DS2 is his polar opposite.
He has recently had a SALT assessment arranged by the paed and later the therapist called me to give me an idea of how he'd faired and she said he came out at the level of a 6 year old for some things and a 16 year old for others so I am rather non-plussed.
If anyone else had been through this assessment, any advice on what to ask would be appreciated.