SEN Legal is apparently phenomenally expensive - I've come across people who have spent tens of thousands with them. Maxwell Gillott is much more reasonable and I believe the results are at least comparable. Both should be able to suggest experts.
I came across a family who spent £15k on SEN legal before they even got a tribunal decision. When I contacted them the first time I was told they won't even talk to you until they have your credit card number. I hired David Ruebain and found him excellent but unfortunately he no longer practices so this time I'm trying to decide again.
If you can face it I would suggest you go to Tribunal on your own. Get support from IPSEA, SOS-SEN or others to prepare the appeal- so you know what the law is that the Tribunal will apply- but spend any money on your child directly. I have been to Tribunal Three time for my child and been involved in very many appeals. Most parents who go to tribunal are not represented and the vast majority win. You need to be careful if you appoint a solicitor as they can be expensive and do not necessarily increase your chance of success. The process is set-up to support parents. If you don't understand something the call the tribunal helpline.