"So if their combined no’s for reception, y1 and 2 are less than 30 per class we have a chance"
Not necessarily.
ICS are the Infant Class Size regulations. It is the law that there is a maximum of 30 pupils per teacher in YR, 1 and 2. So if it is, for example, two form entry and they have admitted 60 pupils, they are full, irrespective of numbers in any other year group.
But if thehad 3x 20 classes in YR, they cannot admit more (even though there would be spare capacity in Reception) if they then have 2 forms in each of Y1 and 2 and can therefore have a maximum of 60 (thus is called "future prejudice'. Actual vacancies in year groups other than the one you are applying for are not relevant. It is only a matter of whether YR is full, or if it looks less than full whetherfuture prejudice applies.
If it is an ICS appeal, the only ways to win are:
a) the admissions criteria breached the admissions code (eg not giving priority to a group they must, or giving priority to a group they must not).
b) the Admissions Authority made and mistake AND that mistake deprived your DC of a place they would have been offered if the error had not been made (eg putting your DC in the wrong admissions category, measuring the distance from your home wrongly)
c) the decision was so perverse that no one could reasonable have made it (the bar for this is very high - things like child protection issues)
If it is not an ICS appeal, the you can also use 'balance of prejudice' arguments, to show how the detriment to your DC from not attending outweighs the detriment to the school and the other pupils if they do join.
Did this school have an 'exceptional medical/social' need category in its admissions criteria? Did you originally apply in that category?