Thanks Neil, worth a go then.

Add the mass as strips or you just make it resonate at a different frequency.. Also If you can make sure there is an air gap. That is a fantastic sound killer. The panel resonates transmits to air inner panel/carpet/door liner has a soft back and is not "energised"

After the failure to reduce the road and engine noise, there has been an unexpected gain - the sound from the speakers (and the sub in the boot) seems to be noticeably clearer.

I guess I'm probably loosing less sound into the body panels

Perhaps that's the way I'll get the benefit - I can turn the music up and hear less road and engine noise....