Just in case this helps others, it's nice to be in a position to give information instead of drawing it from here
In case anyone wants a timeline, the exam was Saturday 9th, ds got a call for an AP interview on the following Friday. We were offered Wednesday but we went for the interview this morning. I don't know what order they are calling people, but based on the other boy in ds' class who has been called, it's not in alphabetical or age or distance lol.
I didn't know what to expect, ds was gone 15 minutes, interview was with a deputy head and another teacher. There were clearly scholarship interviews going on at the same time (so we haven't got one of those lol). Ds was a bit nervous beforehand, but was very relaxed coming out, said they'd just had a nice chat about cycling etc. They had received his report from primary school already as they asked him about an after school club, but it was all very friendly and informal.
The nice admissions lady also told us that there will be three weeks of interviews (this being the end of the first week) so no spoilers before 12 February. And presumably they will still be calling people for the next week or so to arrange the later interviews.