Thank you flowerfairy and amethyst ice for you advice. With your longer journey's and more hamsters travelling, our 4 hour journey with one hamster should be fine
I agree the cat carrier is very large, Tali has got a smaller carrier suitable advertised for hamsters, she dislikes it and she became very stressed the last time she was in it. I'll make enquiries for a guinea pig advertised sized carrier for Tali (thanks for link), as decent size (bigger than our small carrier but half the size of the cat carrier). We'll be stopping on the journey at least once and can offer Tali a drink from her bottle.