Well, their experience is that DIY fitting the amplifier for the arial is not always as sucessful as the dealer doing it, not sure why, but without any experience I cannot argue?....so they basically were saying to use a windscreen stick on arial instead, which in my opinion is a no go as they are ugly things and want to do this the oem way. they would say that as they sell the windscreen arials too ;-)
I think the problem I have found when searching is that there is nobody saying how they have done this, but plently of people saying it is doable. I can get a unit off Ebay for £250 which is a massive saving on Audi charging £800 plus fitting, plus vat....and a saving on the £500 for a non audi retrofit from these guys....even adding the extra bits needed like the fibre cable and the arial amp.
I'm keeping searching, but what I need is to find someone who has done it, then I can find out what's involved so I don;t waste money buying kit which is either beyond a DIY fit or just doesn't give a good enough reception to be worth doing.