Alex
Further to my previous post - and the subsequent replies. The more I think about it, TBS and Tim are probably right.
What I do remember, is that some years ago I was in a similar position - wanting to make sure I got three PE seats on a particular flight. So, what ever the limit was I rang up a couple of weeks before and the agent said she would try to make the booking but didn't think the system would let her.
She tried and 'hey presto' we were booked, which is why I said give them a ring. Whatever the rules say - the computer may say different.
Nick