Thanks for the suggestion of trying the c6 version Steve. From the photographs it certainly looks like they would fit.
But when it comes to fitting parts to the car for the safety of my grandchildren I only want to fit the part that is made for my version of the car and using the torque setting given by Audi - if it doesn't exist then I'll stop there.
I've passed this information on to the parts department of my local Audi dealer to see if they can find the part for the C5 - initially they said there is no part but possibly they were not looking at the right diagram.
It was your initial suggestion that started me on this new path which may lead to locating the correct part so thank you for that. That's the benefit of these forums where people share information.
You'd have thought there was some Audi document on retrofitting the child safety components that Audi themselves supply, especially considering the LATCH laws that exist in the USA.