2.0 TDI Injector Clamp Bolt Thread

Recently purchased a 2013 A6 Avant 2.0 TDI (private), after a few 100 miles one of the injector clamp bolts came loose, have removed this to find about 5mm of thread stripped and intact in the thread of bolt, replaced the bolt with a new one to get the car running and seems to be running fine however I am weary as obviously there is now less thread to retain the new bolt. Does anyone know how deep the thread is for the clamp retaining bolt and do you think I should get this professionally repaired. I am assuming this will be something similar to a helicoil insert. Any help/advise would be appreciated, I am also interested to know if anyone else has had a similar issue and a rough guess as to how much i could expect from a garage bill.