Thanks for all the advice - it is all very helpful. I was however advised by friends who also have a house over there that Saga would be able to insure the car provided it was not kept there on a permanent basis. It seems that many of their insured over-50s have homes abroad and travel back and forth a lot and therefore they are much more flexible and so it transpired. So I now have unlimited green card and no need to notify when travelling - all at a very reasonable £180.00 for the year!! And no problem that the car is a German import Opel - which other companies would not even consider. Thanks again - all information is valuable.
Check your policy as it may contain a clause that the vehicle must have a valid MOT certificate, in which case you will need to bring it back to the UK once a year. The other thing to look out for is vehicle licencing as few countries will permit you to run the car without any form of road tax. You will probably have to register the car somewhere and pay an annual road tax to stay legal.
Had a drink with a Brit last night who has used a UK registered car in Croatia for several years. He can't import it as such and register it in Croatia because of its age, so he keeps the UK plates. He insures it in the UK on a basic policy, to maintain his NCB. As he doesn't take it back to the UK for an MOT, Graham's point is valid. However, he doesn't intend to use the UK insurance for any claim. He has insurance with a Croatian insurer, provided on a month by month basis. All in all, this apparently satisfies the authorities. HTH