4.0 TDI valve rockers broken

[OwwDee]

I have a problem with my 2004 A8 4.0 TDI Quattro.
Started her up one morning last week and there was a strange noise
from the engine bay. I was at my workshop at the time which is 1/2 mile from my house.
I had driven the car from my house to the workshop an hour earlier without any problem.
It was cold, car was covered in Frost.
I gingerly drove it back home, and got my mechanic to come have a look.
We started her up, and it seemed that the left 4 cylinders were not firing.
Loaded her onto a flat bed and brought her to his garage, and on inspection it was found that several valve rockers were broken.

Have any of you experienced this or anything similar?
My mechanic says that he has seen this problem before on the 2.5 litre engine but not the V8.
I will of course be rebuilding the top end with new Rockers, Lifters, and while I am at it new timing chains and timing gear.
This of course is an engine out job as the timing chains are at the rear of the engine next to the bulkhead.

What could have caused this? What else needs to be done while the engine is out?
The engine has 206,000 miles, and she has service history.