Jack, thank you for the this information. I had noticed the small opening and wondered what it was, hence pleased to have captured this in enough detail to show up.
Whilst sorting through the images after taking them last night I came across one that totally baffled me due to what appeared a blown area but knowing that my lighting had not changed and all my exposures had been pretty spot on before and after the shot.
I have now recovered this from the recycle bin, a quick PP, and come to the conclusion that what I had captured was the ostiole opening in action. Fascinating.
Grahame