Hi Andy,
I found the adaptors a lot cheaper to buy over in France from the actual service stations or if they don't sell them most will lend you one.
As someone has already said there is one called a Dutch connector but there is also another type they use which was more common where you screw the bayonet onto the fitting (it's like a large threaded adaptor)this one i came across a lot while travelling down to Luxembourg.
Hope this helps Paul