Further updates, in case of use to anyone going through a similar procedure. We've now had our appt, with Child Consultants (private, as it had to be because the school is): 15 minutes talk to parents without ds there (probably would have been longer if I hadn't sent in an essay about ds in advance), 40-50 minutes of ds on his own with ed psych, 20 minutes debrief without ds. Ds patently loved his time, doing puzzles, getting praise & having a new grown-up's attention all to himself - excited chatter audible all the way down the hall. As well as the verbal debrief, we will have a written report that can be passed to the school.
We were asked at the beginning of the session for any particular concerns to assess & I specially asked that Asperger-type features be looked out for. (Verdict is that because (in particular, presumably other things taken into account) ds could "take the emotional temperature of a situation", managed personal space in a normal sort of way, didn't get hung up on detail, wasn't thrown by figurative language, and (it seems crucially) could whenever asked easily move from one suggested activity to another, that we are not in that territory (although I should not expect him ever to be "ordinary" ). And that a sing-song, slightly too high & loud, every-word-emphasised-the-same speech pattern like his isn't in itself a marker - it sounds silly now but in my anxiety I just couldn't get past that feature, even though I could see that he doesn't always talk like that & that it comes & goes depending on who he's talking to, about what, etc.)
So instead of the diagnosis I was more than half-expecting (and ready for with my next steps planned & research in hand), what we have is some very very high IQ scores & the challenge of seeing whether ds's school can decide to cope. Whole new issue.