Okay, I have just finished replacing the rear sensor and it was actually more straight forward than I had anticipated, thanks largely to everyone's threads in the forum.
Just in case other people have to do this for themselves here's a couple of tips:
1. Use wd40 to loosen the stubborn allen-bolt
2. Line up the wheel bolt holes with the reverse of the rear sensor and use a bit of dowel rod or the flat end of a large nail and them hammer lightly to get the sensor out.
3. When you are inserting the new sensor you'll find that it's a bit tough to get into the hole. I got another person to help and I used the reverse of a large nail (the flat end) put this up to the sensor and get someone to lightly hammer the other end, making sure everything is lining up correctly.
4. You may find that after all of that the ABS light still doesn't go out - just drive it up the road and it should then.
As VW had quoted me around £150 to replace the sensor I've saved myself around £128 from doing it myself. Picked up the rear sensor on eBay for £22.