Hurrorerr Philip, greetings from a fellow Scot. I was down south in Jan and again in May and had a week on Koh Phi Phi and another on Koh Lanta. Alhough I really liked Phi Phi ( thankfully quiet in the low season ) I think it would drive you nuts in the high season. Koh Lanta, however, from what I've been told, will probably still be quiet in the high season. For me it was a true tropical paradise and by far my favourite Thai island I've visited so far ( Phi Phi, Samui and Phangan were OTT in comparison ). Stayed at the Lanta Miami resort in a beach-front (literally) bungalow for 500 a night. There is no nightlife as such though the beach-bar associated to the resort is an experience in itself. Mongo, yes Mongo, no relation to the character in Blazing Saddles, who runs the bar will cater for every refreshment to your taste, booze, smoke, mushroom tea(!). Didn't try the tea myself but had a good giggle watching the younger generation 'spacing out' By no stretch of the imagination could you ever consider the resort 'quality' ( as in 5 star ) but it was certainly no beach-bums' establishment.
For an impression visit
http://home.planet.nl/~afrench/page_01.htm - you'll first see Hua Hin ( PandP Bar ) then Phi Phi and finally Lanta