Found a guide online for converting a US spec MMI to EU and tried that which resulted in a system that only got as far as the splash screen and did nothing. Basically I'd bricked it! Contacted a specialist, f-knaviexpert on eBay, told him what I had done and the result and he told me he could sort it for £200. Took the car to him, he took the MMI unit out of the car, took it apart, did a fair amount of reprogramming in the EEPROM and flashed it with EU firmware, then, rather than waiting a couple of hours copying all the maps over from SD card, knowing that I was coming, swapped the hard drive with one he'd loaded with EU maps. He's in Enfield, North London but said that I could have just posted the unit to him and he could do it and send it back.
The radio had already been swapped for a UK one otherwise he would have done that and said that he can also upgrade to DAB if required. Ordinarily I would have done that but as the car is destined for France where there is minimal DAB, it isn't worth it. If you search Ebay.co.uk for him as a seller, he specialises in Audi MMI units, both replacement units and repairs and reprogramming. Also a very nice bloke and will definitely be seeing him again when I get another imported Audi.