Kwan Thai has good food but is overpriced for what they actually give you (e.g. food is good but not extraordinary). I went earlier this year and the service wasn't that great either. The waitresses were rather grouchy towards us and seemed like they couldn't be bothered, although they were dressed nicely. We sat on the ground floor by the window but didn't get much of a view because the restaurant was slightly sunken so you see more of passerbys than the water.
If you want a good view and exquisite food, try Ozu at County Hall. It's pricey but the food is much more worth that money than Kwan Thai. Mind you Ozu is Japanese, not Thai. The view is amazing though... try to book a table by the window if you could. I must repeat, exquisite food! And romantic setting too.
If you are looking for Thai, I would recommend Lye Torng. It's not a fancy place (pubbish setting with good table service) but their food is amazing. Maybe not for the special birthday meal but worth a try if you're ever nearby!
Thanks for all your replies. They have been really useful for me. I've decided not to go to Kwan Thai, and will keep looking for somewhere which will be better. Any more suggestions will be gratefully received.
Gaucho looked good but slightly more than I hoped to pay and Ozu looked good too but sadly our other friends aren't keen on Japaneses food.
I can recommend Lye Torng for the food. The ambience is not particularly special though they have recently painted the interior a bit and are having a go at the outside which may be better by December. I live locally and go there a lot and keep whingeing at them about their lack of a decent coffee machine, without effect so far. However the food is good and they do make an effort when they have a large party in for a meal. I was there when they had a very good civil ceremony/wedding last year and they do have quite a few birthday parties etc. there.