In reply to gingerkate:
Can I suggest a compromise? From your investigations, it would appear that there is no maximum limit set in stone, simply an advisory.
For an advised limit of 2000 words, 3000 to me might be seen as "taking the piss", even though your young person has not broken the rules technically.
But you say they have valid points to make. So why not suggest to them that they need to rein in, but allow a little lee-way, say to 2500 words?
Teaches said youngster about being concise, but also how to use the rules to their own advantage. I could see a tired and grumpy examiner being rather peeved if they are faced with a 3000 word essay to mark, after completing heaven knows how many already, however 2500 is not stretching it as much, and if the student has completely valid and genuine arguments to make in relation to the question, it will not be as noticeable. (Although an extra 500 words of waffle might not go down as well, if you look at the flip side).
If the exam board are stupid enough to not put proper guidelines in place, exploit them, but don't take the piss, IMO!!