All resolved v quickly - as hoped, appears to have been a misunderstanding (well, a couple of misunderstandings). Teaching assistant called me shortly after seeing my email, and said she HADN'T crossed out any other answers, just marked that one answer as wrong (which he knew it was anyway). So his misunderstanding there.
She does seem to have told him he was 'cheating' - as the children were told clearly beforehand not to talk during the test - BUT she didn't realise he had already finished.
Hopefully no harm done - she said she'd tell him now that he hadn't lost any more marks than the one he knew he'd got wrong.
I'm happy to have talked about it with the teacher concerned rather than playing a game of Chinese Whispers with a teacher who might not even have been there yesterday (there are two part-time teachers, so the teacher I talked to today probably would not have been in the classroom yesterday, and would have had to go back and forth checking things with the TA).
Hope ds is now OK about it - he sounded upset yesterday at being accused (unfairly) of cheating, but if he understands it was a misunderstanding, and that in future 'big tests' he shouldn't talk until the teacher actually has his paper, hopefully he will be fine. :)
Hate exams for little ones... Six is very little to have the pressures of grown-up style 'exams'. :(