Just posting this for info.

My 1999 A6 2.4 suddenly died one morning about 1 sec after starting. On trying to restart it sounded completely different as the engine was spinning without compression. Oh no - cambelt failure - I thought. I got my local independant garage to recover and they agreed with the diagnosis and stripped the front of the car down to get to the cambelt - which was intact.

It turns out my oil pump pressure release valve had stuck closed, over-pressurising the engine and causing the hydraulic tappets to misbehave - resulting in the lack of compression.

It still cost be 1000 to fix but at least I didn't have bent valves & dented pistons and I now have new cambelt kit and water pump fitted. Engine now running sweet as a nut again.