After a lot of faffing about I have managed to set up a calendar with one daily session for up to 40 people. The issue is that 2 days of the week are showing that they are full after only 4 people have signed up. All the rest of the days are fine and I previously tested that it would allow 40. I can't see any differences between the set up for these appointments as against the other ones.
I have tried extending the availability from 40 to 80 on one of those days and it makes no difference. Somehow it seems to think that the session is full