UK Car Repair: Hydraulic tappets, range rover v8, hydraulic tappets

Question
What could be the probable causes for the exhaust valves not closing fully in an engine fitted with hydraulic tappets as fitted in the Range Rover V8 3.5 litre engine. This is exactly what is happening in my engine, the valve seats are all black.

Answer
Bent or sticking valve stems would be my first thought but not if it is all of them. This also eliminates broken valve springs. Where are you located? could it be thick oil in a very cold climate locking up the lifters?  Clearly the engine must have been running very badly, did this just start happening?