Thanks to everyone for their suggestions.

I'm going to pull it all apart at the weekend, and see what improvements I can make in the wiring, and it's positioning.

Unfortunately, the space behind the dash is pretty limited, so I'm not sure what I will be able to achieve.

I'm also going to speak to the car hi-fi shop who sold me the antenna adapter, and see if they've ever experienced problems like this.

Thanks to all.

Andy.