Well after a lot of head scratching I decided to start from scratch with the the car aerial, I had a look at the antenna base and thought this was the suspect, tracking a second hand one down was quite hard as they seem to fly off the shelf. New ones are approx £60 ex VAT. Not sure that this was the problem, I decided to check the cable first.
After moving the electric roof pump (see below) I removed the nut that holds a ground wire and the car aerial cable, upon removing this I noticed a spark!
With the cable removed and disconnected at both ends I tested the cable for continuity by using a scrap of cable as shown below.
I found that there was a short circuit in the cable with the small wire in position and when removed!
As I was in the car with the tester I examined the aerial connector that plugs into the car radio, I found that the centre wire of the coax was not connected to the centre pin and shorting out on the ground of the connector (I now understand why I got a spark from the aerial cable when I disconnected it in the boot, the antenna base has a 12vdc supply when the radio is switched on).
The picture below shows a connector with the black plastic shroud removed.
With the centre wire now connected the radio reception is excellent and works as it should.
