In reply to Rog Wilko:
I can't help with an answer but I'll be interested if you find a solution as we are likely to be in the same position in a few weeks. Please update the thread if you find an answer elsewhere.
My sister and her partner will be visiting from Australia. UK citizens but they had to swap their UK licences for Australian ones after they had been over there for a certain period of time.
Haven't looked at putting them on my van insurance yet but I know they've looked at insurance on a car and found it was going to be prohibitively expensive (I forget the exact figure but I think it was a few thousand for a few weeks!).