I thought I would have received some kind of reply by now.
Have I raised this issue incorrectly or does nobody know?
I have been investigating and am guessing that the aerial is in the tailgate. after I removed the trim I found an aerial amplifier. Checked connections and can't see any problems.
I thought I had better check that I could get the trim back before going any further. I have now knocked two of the retainer clips into the tailgate pillar and can't get them out. I can hear them sliding up and down but can't get to them. Will drive around for a while before having another go, hopefully the clips will have worked their way down and I might be able to get them out and then check out my original problem of the aerial with a meter....