sounds like its TIME to remove the cylinder heads, it maybe a siezed engine bearing,(a guess)!
pic of what the chain and tensioner look like , BUT a lot of work to get at it!
ron

Jag V12 chain and tensinior

 

You can take the oil pressuere sender off at the back of the V.
Then rig up a container to fit.
Fill the container with some thin oil.
It will dribble down the oil galleries and lubricate the bearings and camshaft etc.
More likely the rings are seized, but everything helps.