Any letters you send from doctors etc need to be photocopies as they won't return original documents, and look online for organisations that offer guides for filling the form in, cerebra were excellent when I applied, my daughter is being diagnosed for asd. Ultimately it all comes down to showing what extra care your child needs , not what their diagnosis is. Dont rush the form, you'll have six weeks to fill it in, and photocopy the whole thing before you send it off, just for future reference, and send it recorded delivery !
Good luck