Docs advice is sound and you can get it cheaper on ebay but I am wondering whether there is any change from the factory fit and my suspicions are that there may well be.  I have costed it and it would cost me the following to get it sorted via ebay:
£650 for the version C DVD unit - ultimate velocity in Leicester seem to have a few units
£265 for the factory fitted iPOD option and front centre armrest
£100 - £150 to get it fitted
so all in all around £1000 which is an £850 saving over factory....
however:
- you don't get a warranty with the ebay unit - 3 years with factory
- if the antenna is installed under the dash then does that mean you are stuck with the bee-sting aerial (sat nav fin looks nice - petty, I know but I am planning to keep this car)
- is there any difference between the UK units and foreign ones?  From what Deva says, there is... although it seems to be an improvement over the UK unit.  However, if there are differences then maybe there are certain things the UK unit can do that this other one can't?
- Does everything work as the factory installed one, i.e. multifunction wheel etc..?
I am already spending around £26k so am thinking should I try and save £850 - still a lot of money - and go the ebay route?  Really depends on answers to the questions above.