Where to get a private assessment for autism & ADHD in London?

Hello - I've just joined today in order to get some help for my 18 year old daughter. She's on a waiting list for an assessment on the NHS but this is such a crucial time in her education I don't think we can wait for the year or more it will take. The GP has recommended we get a private diagnosis. I've looked at the directory on this website but what I really need is a personal recommendation of a diagnostic service that is good and hopefully reasonably priced. The directory allows anyone to advertise so there's no way of knowing about quality or value for money. Or maybe they're all as good as each other?! I have no idea. Any advice very welcome! Thank you in advance.