I got the horn problem :)
Just as a heads up for others really, my Cab went in for a service at Audi this week, before it went I scanned it with VCDS Lite as I noticed when the alarm went off the hazards flashed but no sound. Showed up H12 siren short to ground fault.
Interesting to note that when I collect it from Audi they didnt even mention the fact the alarm wasnt working or the fault code. Im rather dissapointed given its a main dealer etc.
Took the siren out today (dead easy on a cab takes 2 mins!), contacts were very corroded which rang alarm bells, cleaned them and put it back in and tested. Nothing.
Cracked it open and found what I wasnt hoping for....so EBay here I come, bugger is I just missed out on one on ebay for £25 litterally 20 mis ago it sold whike I was breaking this one open.l