Thanks for all this advice. It's very interesting. Yes I first started playing as a kid about 40 years ago, but have mostly played jazz and swing, and I hardly played at all for many years. The cramps have only really started in the last 5 or so years, since I started doing Rockabily. String tension is very low as I'm on gut D, G and wrapped synthetic E, A. But the action is quite high, much higher that one jaxx set-up bass. If I'm slapping them I tend to get entangled in the strings if the action is too low Also as a jazz player, I have found the technique pretty unnatural. I like it, but haven't found it that easy.
Maybe I should look at my action in a bit more detail.
Thanks again.