Right, finally got this sorted after much phoning around! So basically the only option, should you find yourself in this situation, is to use a third party insurance provider to provide transit cover.
There are a few of these companies about but seemingly only one that will cover goods in transit from one private individual to another. The company is called theinsurancebroker.com
Tò give you an idea of the cost, for £4000 of coverage shipping to Europe, it's £65. For the rest of the world, £93.
Not cheap I know but actually not too bad when you consider the alternative...
Hopefully this post will prove helpful - I just wonder how many people are shipping their basses from the UK not knowing they are not even covered by most couriers insurance???