It was really small, quite mobile, and I'd knocked the lens to f/5.6 so lost some depth of field... Just checked the markings on my ruler and it was probably only just about 1.5mm at most!
It might be a Deuterosminthurus repandus - going off the Ocelli in characteristic arrangement: upper row 3 ocelli, middle row 1 large ocellus, lower row 4 ocelli.
Tight crop of #3
Paul.





