Can anyone tell me what is used to release excessive pressure in the cooling system on modern(ish) cars.
My old cars used to have a radiator cap with a spring pressure release valve built in bu my Audi A6 does not appear to have any such mechanism.

It should still be in the cap.

VAG cars have strange caps on them but the valve should still be in there just maybe not as obvious as the older type..

On most modern cars the pressure relief valve is built into the expansion tank cap. This consists of a rubber seal that is spring loaded, just like the old rad cap type.

Agreed - all systems need a pressure relief valve somewhere. It's the only way to stop your cooling system exploding if you overheat