It appears the information being given out was not up to date, and this is clearly unsatisfactory.
However, maladministration in this context would mean that the authority made a mistake as a result of which
the child was denied a place to which he/she would otherwise have been entitled.
Sometimes this sort of thing is easy to prove objectively. For example, in a case involving oversubscription where distance is one of the criteria, if the authority has incorrectly measured the distance between home and school, the correct measurement can be compared with that of the last child to be offered a place under the distance criterion, and there is no difficulty in determining what should have happened.
In your case:
1. It is not clear what would have happened - you might have benefited, but so presumably would other applicants. It's not going to be possible to prove your son would have been entitled to a place.
2. The missing bit of information is almost certainly unlawful! No wonder they dropped it - admission arrangements must be objective, and not open to subjective judgement.
You could simply ask a question about this at appeal, but I think it would be a big mistake to focus on it.
Your grounds for appeal should be:
1. Alternative academic evidence of high ability:http://www.elevenplusexams.co.uk/appeal ... cation#b11
2. Reasons for wanting a place at this particular school:http://www.elevenplusexams.co.uk/appeal ... -school#c2