Long time lurker but thought I'd register to answer this...
I'd try the Thai Silk just off The Cut, in the arches just behind Southwark Tube. It's relatively quiet compared to the one near Waterloo station, has a very pleasant atmosphere and the staff are always absolutely great. They're very polite and gentle and it feels much more calming than a typical thai restaurant in central London. The trains do rumble overhead from time to time but I hardly notice it!
Oh - almost forgot - the food is very good for the price.
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