Thanks for the feed back. It might be dehydration. I'm off for some sports massage tonight, so I'll ask him to check out my legs.
Carl, the best way to learn tumble turns is to get a coach. You can't see what your doing wronge if you try and learn from a web-site or book. Most clubs have either adult swimming lessons or Master Classes. If you contact your local pool, there will be someone there who can teach you.
When I first learnt to tumble turn (and we still do this as a warm up exercise prior to tumble turns), we had to swim just four lengths of the pool front crawl, but on every 5th stroke do a forward roll and then keep on swimming. You just have to remember to breathe out of your nose, other wise the experience isn't very nice. But being able to tumble turn is well worth it especially in time-trials and for building up pool endurance (you don't get any time to rest between lengths so you have to push your body harder and think more about your swimming technique).
Talking of technique, I'm going to invest in flippers next week, can't wait!