In response to Bottle, I have just put my multi meter in DC current mode between the aerial plug outer (which I have previously proved had continuity to the roof and on which there is a 12v potential between it and the radio +ve feed).
With the CD playing or with all the lights on (shows 35A draw on the ammeter connected to the shunt in the battery -ve lead) there is 0.00 amp draw, indicating that there is no stray current going through the hull via my rubber duck car aerial.
As radio reception is excellent, I shall leave it as is.
Thanks all for advice.