You think we should mention that she has adhd in the appeal letter?
Yes, especially on 113/105. Don't overdo it, because it does sound as though you have got strong academic evidence, but on lower scores you really do also need a reason as to why she couldn't demonstrate her ability on the day.
Can they discriminate against us for this ?
Most definitely not. In fact, the panel may feel that they should consider whether your daughter has a disability under the terms of the Equality Act: http://www.elevenplusexams.co.uk/appeal ... cation#b33
her HT has confirmed that there was also a dip in her school work at the same time
This will need to be borne out by evidence, and more detail from the Head and also the class teacher would be helpful.
You might also want to find examples of where the "dip" is evident in her exercise books - good work from Year 5, poorer work in the early part of Year 6, and then an improvement when her condition stabilised. Although schoolbooks are usually presented to the panel at the end of the hearing, you might like to submit copies of 3 sample pages that demonstrate the problem with your appeal evidence. You can refer to them as Appendices A, B & C in your letter, and mark them as such.