I'd recommend Ratchada on Lee High Road (just through Blackheath Village)
http://tinyurl.com/32asfq5
No decent Thai restaurants at all in Greenwich. Thai by the post office in Blackheath is also quite popular.
Actually tried Thailand in New Cross (think it's opposite Goldsmiths? We got delivery), really great curry, but really not worth the money on starters. And well worth sharing rice between two - expensive portions, double the size they need to be. BUT, now I know the curry is so good, I'll order from there more often.