Thanks for the reply.
The usb leads form part of the stereo wiring loom so I can't try a completely different cable. I have tried a different port on the stereo with no improvement though.
I have nothing plugged in to the cigarette lighter port and no LED lights, so those can't be a problem.
The problem persists with ignition on / off and engine running / off also. It's there all the time. I did try a temporary extra ground cable and this didn't help.
I'm now thinking that I need a more expensive adapter, but I have no idea which to get. Reviews seem very sparse. I guess I could also try a roof mounted aerial, but I really don't fancy drilling holes in the roof.
If anyone has a suggestion for a better adapter or a amplifier or any other suggestions, it would be much appreciated