I'm in Thailand right now to train Muay Thai for a year, but I've also just started Thai dancing classes for something to do in my spare time. This involves some stretching of the hands so that you can bend your fingers backwards etc - do you think obtaining unnusual flexibility in your hands for Thai Dancing would cause any problems for making a good strong fist etc for Muay Thai? (or indeed having a good grip for BJJ etc)

Thanks in advance for your advice

Lucky you! Your hands can be both strong and flexible. I would work on grip exercises to maintain and improve hand strength while you do these stretches.

As long as you maintain your grip/finger strenght, I don't see a reason for the extra flexibility to be detrimental. I would pay attention to the muscles that extend (open) your hands by using rubber bands just above the fingernails while flexing your fingers/hand. That's one aspect of hand strenght most people neglect.