Getting output through RF but not through composite cable

[RussWWFC]

Hi

I bought a dreamcast secondhand a couple of weeks ago, and all worked fine on my current TV which has an analogue tuner. But the picture was horrible, all washy colours and so I got myself a composite cable

But it doesn't work, tried it on 2 TV's.

So is the cable faulty, or could it be something else. I have had a look on the settings of the dreamcast, but can't find anything obvious. It's a UK/European Dreamcast if that makes any difference

Thanks, happy that I am getting back on the dreamcast scene and chatting on here