What is it you need? Some ingredients available from Fusebox in Borough Market and unexpectedly the new M&S Simply Food on Southwark Street has quite a few items. There is probably somewhere more authentic (and cheaper) down Walworth/Old Kent Road though.
Many thanks for the myriad quick responses - I will check them out. Responding to boroughpaul's quest, Tesco Surrey Quays has dried galangal and kaffir lime leaves (small quantities and expensive). Did buy but have not used them yet so no comment on quality. I was intrigued by 'holy basil' in the Thai recipies I was wanting to try, but my Thai girlfriend seems to be happy enough with the authenticity of Tesco ordinary basil when we tried it out in a Gai Pad Bai Graphao a couple of hours ago.
I also found an Asian Supermarket on Evelyn Street this afternoon, presumably the one you refer to jantheoldone. Pretty good - it had the brand of Naam Plaa that my girlfriend uses back home, lots of fresh lemon grass and some other Thai leaves (vegetable not herbs) that we will go back for again. Nice fresh Pak Choi too (ok not strictly Thai but they love it). They also had fresh galangal, which she thought was a bit old looking, hence the dried stuff from Tesco. Curiously no oyster sauce but that's no problem sourcing from Tesco. Tesco also incidentally had Papaya if anyone fancies their hand at som tam.
So it looks like a combination of the Evelyn Street supermarket and Tesco SQ will keep my experimentation going for some time
Don't forget the Hiep Phat shop at 233 Walworth Road. It is a Chinese and Vietnamese grocer, but they also have Thai stuff. Staff don't speak much English, but look in the freezer cabinet on your left as you go in: it is chocka with frozen fresh Thai basics.
That Chinese grocer next to Asda is a bit grotty and doesn't have much fresh stuff.