Thanks Rog, really helpful.
I found the front spoiler on eBay. I bought a whole front bumper with it on and cut it off, then had it resprayed and fitted. You have to really look for them carefully!
The rear window spoiler was already part of the S-line spec. Again should be able to find an original one off eBay. The bottom rear diffuser I also got from eBay. I spent a long time searching! It is the Votex kit which is made for Audi.
If you do a search you will get all the info you need. Be carefull though to get the right parts for your type of A3.
Hope that helps mate,