400 quid is a lot of money for Bangkok. You can have just about anything you want. Depending on what 'style' of hotel you like I would suggest either The Oriental or Peninsula.

IMHO The Peninsula is a little more contemporary...and appeals to a younger crowd than The Oriental. The Oriental is what I describe as a traditional 'refined luxury' hotel. My partner and I love The Pen, my parents swear by The Oriental. Every room in the Pen has a great river view(I like watching all the river traffic). The same is not true at the Oriental...although they, of course, are directly across the river from each other.

There are so many great hotels in BKK...but these two are almost always mentioned as a couple of the very best.