I had this happen over the weekend, the drivers side latch would not open.
I read all the forms on here and elsewhere and started with removing the inner wing and accessing the junction box to see if pulling directly on the cable that operated the drivers side latch would do it but alas no.
I did revert to using a saw under the bonnet to cut the clamp, took about 2 mins to with minimal damage to the bonnet.
This thread covers off how to do it with pics
http://www.audiworld.com/forums/a8-s...830934/page11/
The root cause was the gland/ball at the latch end of the cable had come adrift so the cable just pulled through, likely a combination of corrosion and not being lubricated.
Today I visited Audi Edinburgh prepared for a touch your toes price but the cable in question was less than £8 with the catch bracket less than £5, I have both cables and bracketry on order. as I think it prudent to change both.
I will put the Audi part numbers up here shortly.
For £15 quid its probably worth changing your cables and lubricating the latch mechanisms if you feel bored one afternoon