Just back from dropping Iris off and managed to speak with the room supervisor/preschool manager and the special needs teacher.

It seems yesterday's incident was triggered after a member of staff said it was pack up time. X wasn't tidying away the lego and Iris took it upon herself to help; she's going through a bit of a policing stage at the moment, whereby she enforces the rules. (Callum normally bears the brunt of it, poor thing.) X, being autistic, has a special way of putting things away and really didn't appreciate Iris interfering, so bit her. So neither child is in the wrong, as such.
Anyway, the staff were very apologetic and assured me that they're going to be spending lots of time with Iris. They've also hired a new member of staff, so that should help the situation.

(The ratio over here is 1:10 for her age group, so they already exceed that and will do so further with the new person.) And they are going to be more diligent at trying to keep X and Iris apart a little during group time, so she doesn't bear the brunt of X lashing out. (I get the impression that while X is indiscriminate in who he bites/hits, Iris gets more of it by way of her being where she is developmentally - the mini gestapo thing).
So I am feeling a bit more reassured. FC that it doesn't happen again.