LL
Yes, an external aerial sited well away from the offending radio would be the obvious solution. Trust me to buy a GPS set that doesn't have an external aerial socket!
Am going to put a bit of pressure on the radio manufacturer as several people have suggested that this set may very well not be inside the EC's electro-magnetic emissions standards which are now mandatory and which would threaten their CE mark on the product.
A bit of 'naming and shaming' might well follow!
Anyway, if I ever get to a solution I will post here (if anyone is interested) and once again many thanks to all the ppruners who have taken the time to make suggestions.
jetfour