State your case quite simply.
An example of a simple letter of appeal can be found here:viewtopic.php?f=35&t=15025
(scroll down to no. 14)
I'm afraid the only sure way of claiming maladministration would be if the letter had been sent by recorded delivery. Assuming it wasn't, the only thing you can do is to attach a letter from the referee, confirming that the reference was sent, plus a copy of the original reference.
You might improve your chances if you also state some good reasons for wanting a place at the school. See:http://www.elevenplusexams.co.uk/appeal ... -school#c2