Just an update.
We have had DS's transition conf and it all seemed to go really well. We had a long questionnaire to fill in with current and next school about his learning style, his strengths and weaknesses, triggers for behaviour, ways of dealing with them, sensory issues, communication routes etc etc.
The secondary school had a transition plan for up to the summer holidays with visits planned in, what each visit would cover, who needed to come.
They answered all my questions and were really positive. They even have their own toilet off the learning support corridor!
They'll do a buddy scheme if necessary, for things like the bus journey home, and they have staffed lunchtime and breaktime clubs in their dept for those who need/want it. (With a Wii!)
They have a computerised passport, which each teacher has access to on their laptop, which comes up when they take the register for each lesson so it's not as if they can pretend they haven't seen it. They are happy to put 'teacher or TA to fill in H/W diary.'
I met the parents of another DC with ASD who will be starting the same time. The school only have the 2 with statements for ASD starting in Sept. Others on SA+ obviously, and others with different needs, but he will get known, hopefully.
I'm actually really impressed. 
It's the advantage of a big school that they have all this stuff as standard. The disadvantage, of course, is that he'll be one of many, and may get lost in the system, but they seem very proactive.
I blew it a bit when I had a little tear about him leaving his class of lovely, tolerant peers, but I think I got away with it. 