Have you sprayed it (repeatedly - may be needed over several days) with a penetrating lubricant. Also you could try a ball joint separator (if you can get it to grip and hold). Where are you located (in regards to getting a new bolt) - I can make some suggestions, but your profile name shows a UK email.

 

Hi,
Many thanks for such a quick reply. I will as you suggest repeat use of the penetrating oil over several days. My other alternative is to drill it, but concerned I might do further damage by straying into the alloy arm. Yes, I am in the UK and I am guessing you are in the USA. Do you know what length and gauge and quality of steel is used in the bolt and I shall go to the local ironmongers and get a suitable replacement. Funny thing is I have just changed one on my much older Mazda MX5 and it was easy and without incident.

 

Do you have any Audi dealerships nearby. In the US they are more expensive - but for a bolt or two it really doesn't set you back too much.. and in a pinch (pun intended) it is worth it.

You can also go here to get info on parts.. For the pinch select the front axle/steering after selecting the year/model of your car.

https://nemigaparts.com/cat_spares/etka/audi/a4/

 

Found this too. Audi Parts Direct | Genuine Audi parts and accessories