replacement / after market HU

No there aren't apart from some Blaupunkt units but you have to pay £70 for the lead, totally pointless.
 
also whilst on subject, ive decided what headunit to get, only things im not 100% on are which ISO leads, i need, think these are the correct ones, the cr doesnt have bose, but has the little sub and amp in the rear corner of the boot

http://www.caraudiodirect.co.uk/audi-wiring-adaptor-speaker-input.html

info:
This adaptor will enable you to replace your factory fitted stereo with an aftermarket unit, and connect it to your factory fitted 'Active Speaker System'. Fits the following models:
Audi A3 without BOSE system (Oct 1996 on)
Audi A4 without BOSE system (Feb 1995 on)
Audi A6 without BOSE system (Jun 1994 on)
Audi A8 without BOSE system (Sep 1994 on)
Audi TT without BOSE system (Mar 2000-Feb 2007)

and radio surround:
http://www.caraudiodirect.co.uk/audi-facia-adaptor-5.html

This adaptor will enable you to replace your factory fitted stereo with an aftermarket unit, and retain the original appearance of your dashboard. Fits the following models:
Audi A4 (2001 X - 2007)

and finaly do i need any fancy radio aerial adaptors, there isnt a roof mount one, only the ones on the rear side windows

cheers
 
That adapter is for speaker input which is a horrible way to feed the amp, you need the RCA input version which is a straight feed to the rear amp. It's this one although CarAudioDirect still have the pictures round the wrong way. There's no difference between the Bose & non Bose other than the Bose one gives you front & rear RCA's, either will work.
This adaptor will enable you to replace your factory fitted stereo with an aftermarket unit equipped with RCA plugs (front & rear), and connect it to your factory fitted 'BOSE Speaker System'. Fits the following models:

•Audi A2 with BOSE system (Aug 2000 on)
•Audi A3 with BOSE system (Oct 1996 on)
•Audi A4 with BOSE system (Feb 1995 on)
•Audi A6 with BOSE system (Jun 1994 on)
•Audi A8 with BOSE system (Sep 1994 on)
•Audi TT with BOSE system (Apr 1999-Feb 2007)

And you will need the antenna adapter which feeds power to the antenna amp in the rear.