Zinc, have you looked through this thread yet:
http://www.a2oc.net/forum/archive/index.php/t-13817.html
It gives some pointers to the fact that an 0818 fault can sometimes be related to a compressor that's about to fail.
Whilst you've done the 'spin test', it is only an indication of the inside state of the compressor and from what Humps said on the above thread, a new compressor fixed his issues.
So, you could try this route first - I've had a re-manufactured compressor fitted in the past (in fact I fitted it myself - not a tricky job, but the re-gassing is obviously a specialist-only task) and it's still going strong 5 years later.
They can be had for less than £200 - for eample:
http://www.ebay.co.uk/itm/AIR-CON-C...ts=Car+Make:Audi|Model:A2&hash=item2577801f47
No link to the seller of this one and no inference that it's the best option either.
Cheers,
M
