If you don't do so already, display a copyright notice on your next newsletter, and an explicit request not to republish without permission (word it however you want). If they do it again, you might have a stronger case, though I believe that in the UK copyright is implicit, unlike the USA where (I could be wrong) it is necessary to assert copyright explicitly.

If you are quite clear that you don't want them to copy and republish your material, I agree with Burke that you should try to sort it out with them directly.

I'd suggest you should follow that course for a while, making sure that you keep copies of all correspondence in case you need it for proof later, and see what happens. Then if they persist, get some legal advice. Our anonymous guest has a point, that you may not get much compenstation by legal action because you have to prove some sort of loss of income. If you are doing this all for free anyway there's no income to lose, unless your newsletter carries adverts and you can show that your are losing advertising revenue.

One possibility is to state in your newsletter what Splash FM are doing. Be careful not to make it libellous, but you can safely state in print that you have not given them permission to use your material.

Anahata