Radiator Leaking from Rubber Bung?

[woodpoleman]

I have a radiator leak that is coming from what looks to be a rubber bung located underneath the left side (passenger side) of the radiator that is nearest the engine. It only leaks when at pressure and won't come out by hand so reluctant to wrench it out without a replacement.

I can't find any drawing or reference to it in the works manual, local radiator repairers don't seem to know of this and Audi parts are not aware of its existence.

Has anybody else had this problem and if so what was the solution?

Thanks