She did it!
I am so unbelievably proud, this probably sounds really stupid but it's such a bad phobia and has been going on so long.
I really thought she was going to back out. I felt quite anxious but really tried to keep it to myself. We were shown in, met the lovely dentist who dd had met last time, she offered dd either a nasal spray or drink to start the sedation process. Dd chose the drink which she had then we sat around chatting until it kicked in. When dd started getting drowsy, she sat in the dentist's chair, they checked her blood pressure and then put the needle in her hand. I felt so emotional seeing her in the dentist's chair which she has not sat in for a decade.
The dentist was amazing, had a lovely manner, even though dd was out of it, she spoke to her at each stage, explained what she was doing etc. She examined dd's teeth and gave them a thorough clean. Amazingly, even though dd hasn't had a dental check up in 10 years, her teeth were healthy with no decay 
The dentist then asked me and dh (who came too) to wait in the recovery area and then they brought dd round who was walking very wobblily! We sat there for half an hour or so, dd got hiccups and had double vision which apparently is common. Then we left, got a taxi (dd fell asleep again!) and we got home.
She said it was really weird, she thinks she remembers some of it but it feels like a dream. She said immediately afterwards she could do it again 
fluffy dd also had her hand taped up during recovery so she couldn't see it.
I am so grateful to the wonderful team at Guy's; after everything we have tried, all the many many professionals we have seen over the years and now dd has made this huge step.
The next stage will be tricker - orthodontics which dd is very keen to have. She obviously can't be sedated for that what with 6 weekly appointments but we will take it slowly and see what happens. The dentist is now going to refer dd to the CBT department at Guy's; dd has previously refused any talking therapies but she might give it a go now.