Currently going through Tribunal appeal for the same thing for the same reason.
1st Tribunal was adjourned while waiting for yet another report from J's consultant which arrived today. New Tribunal not till January
I have loads of info about this and have written lengthy letter to them about it which you can have if you want - if you put your email address on here, I'll send it to you.
The key issues are:
severe behavioural problems
and
arrested or incomplete development of the brain (which the DWP now accept autism as being) with severe impairment of intelligence and social functioning
All this is called the severe mental impairment rule.
As Jimjams says, Megarry means that they have to look at 'useful' intelligence, not just IQ. I argued that, although J does not have severe learning difficulties (which was the main argument from the DWP, even though severe learning difficulties are not mentioned in the criteria), he functions as though he does at times, as he is unable to apply his intelligence usefully in real life.
It's a legal minefield, apparently - the Triubnal service got DWP to send one of their senior people to my tribunal to contest my claim and he said that these are the most difficult cases as it's down to your
definition of intelligence.
Good luck!