Thanks Kerry, yes I've found some 1 and a half day tours that run to Khao Yai from Pak Chong that look good. Great to hear you've had good experiences.
Weirdly enough there's actually been a three-part documentary about the wildlife of Singapore on Sky 1 recently, so it doesn't sound so weird! I was thinking of getting some flights via Singapore, so there's no reason I couldn't stick around for a few days. Thanks for the tip!

