This school is playing fast and loose with the infant class size regulations. They do indeed have a PAN of 21. It appears they rely on natural wastage to ensure there are no more than 60 pupils in infants.
According to the school, the information the OP has been given about class arrangement is wrong. They actually have 2 classes covering Reception, Y1 and Y2. Provided they have 60 pupils across the three years that gives them two classes of 30. These meet in classrooms with nominal capacities of 27 and 29 children.
They then have two further classes, one for Y3&4 and one for Y5&6. These classes have 40 and 41 pupils respectively in classrooms with a nominal capacity of 28 pupils. That is a lot of pupils in each class.
Their official capacity is 133 which would suggest a PAN of 19. If you add up the figures for the individual classrooms it only comes to 112. I think they are slightly understating the capacity of the classrooms but overall the picture is of a school that is packing in as many pupils as it can. If every year was full to capacity they would have 147 pupils. I note, by the way, that the school says it is currently at net capacity which is incorrect. According to their figures they are 8 over net capacity.
So where does this leave us? Is it an infant class size case?
On the figures for September the answer would appear to be yes as there will be 60 pupils in two classes covering infants. However, I think you can argue that, by setting a PAN of 21 (which, their website states, is for every year, not just for this year) the school is planning for 63 pupils in infants. They cannot assume that natural wastage will keep them the right side of the regulations. They therefore have to employ an additional teacher if all three years are full.
Admitting an additional pupil to Reception in September means they would have to employ an additional teacher immediately, but they will have to do that anyway if they take 21 next year and no-one leaves.
As things stand, if someone moves into the area and applies for a place in Y1 or Y2 for September they would have to refuse the application to stay on the right side of the regulations even though they are currently below PAN in both years. In that situation an appeal panel may decide that, because they are below PAN, they must admit.
I would argue that the reason the infant class size limit would be breached is that the school has set a PAN that means it will be breached if all three years are full, not because your child is admitted. I'm not saying the panel will definitely accept that argument but it is worth trying.
I will PM you about your case.