It does sound like an inlet valve problem, outlet valve issues tend to create 'popping' sounds on the inlet side and odd MAF / MAP readings. As GraemeS has stated, it's very unlikely to be an oil supply / pressure issue.

So to summarise is it most likely a head off, replace valves sort of job? What causes the valves to fail?

If replacing one cylinder’s set of valves, would it be prudent to change all the valves in that head?

I think that you're getting ahead of yourself. Something seems amiss with a valve, perhaps stuck open and perhaps the head will have to be removed but there's no reason to suspect that there is an issue with all valves. A damaged piston is a possibility if an injector has been leaking for a considerable time but perhaps there's just a damaged valve rocker too.

As above, it doesn't have to be 'all-or-nothing' to diagnose & fix, I'd suspect premature wear but it doesn't have to be a 'head-off' job to determine the cause nor necessarily to fix it.
A cylinder leakage test would be my next step, if it passes that, then it rules out lots of possible causes and allows you to concentrate on probable ones.

Have you got a stuck, or mal-functioning EGR valve? They can fail, I have one not operating on Bank 2 on mine to have a look at.

That wouldn't cause compression to be down on one cylinder (Cyl 6, posted above).

You are right if it is indeed a stuck or headless valve. It also not unknown for EGR valves seize and also leak fluids into the intake as well. If it were me I would have the heads off. One has to start somewhere, and proving the mechanical integrety of the engine is usually a good start. THe shear cost is the problem of course, as it can force one to scrap a vehicle these days due to few people also having the skills to do the job. I used to be keen on Jaguar V12 engines, but current costs of over £3000 for a set of pistons and liners, plus the agro of getting corroded heads off the studs is the reason very few people will tackle engines these days. Findin broken piston rings for instance would then mean a total strip down as the oilways will need cleaning out.

If I tried that 'brute force' diagnosis on engine running issues as a first step, I'd soon be out of a job.

Yes, it may come to the point where the cylinder head has to be removed, however, there are a few external tests & checks to be done before that is inevitable.

jagracer wrote:

Have you got a stuck, or mal-functioning EGR valve? They can fail, I have one not operating on Bank 2 on mine to have a look at.

Phoenix wrote:

As above, it doesn't have to be 'all-or-nothing' to diagnose & fix, I'd suspect premature wear but it doesn't have to be a 'head-off' job to determine the cause nor necessarily to fix it.
A cylinder leakage test would be my next step, if it passes that, then it rules out lots of possible causes and allows you to concentrate on probable ones.

How far would you go yourself if the head has to be removed to replace a possibly burnt valve? If you remove the inlet manifold but need a workshop to R&R the head, would you refit the manifold and other parts to make it driveable? If a valve is burnt then it could be due to a leaking exhaust manifold gasket so the manifold may have to be machined, although that would only be a minor extra amount of work.

Personally I wouldn't commit to removing the head until I'd had a good look at the valve gear - and only then with a firm plan to put everything right (source of replacement valve(s) seats, gaskets and tool hire of alignment and 'special' tools) If the valve gear is OK, then I'd be smoke-testing to check for exhaust manifold leakage as you may well need additional parts or engineering services to correct.
Time spent on comprehensive diagnosis is never wasted and will give you the best possible chance of a first-time fix, however big a 'fix' it turns out to be.

A tool is supposedly needed to align a 3.6 inlet manifold but there was no need for such a tool when my brother replaced an inlet manifold on the 3.6 in his L320.

Quite often, the 'special tools' can be replicated from stuff lying around once you know what it does - the one you mention is effectively just a bolt with an over-sized shoulder, easily fabricated from one of the old manifold bolts. It's purpose is to centre the manifold over the tapped holes to prevent the anti-crush spacers from damaging the gasket, once you know that, it's an easy task to either make one up or just take a bit of care fitting the manifold fixings.

We are having to change quite a few cylinder heads (non- JLR) due to burnt exhaust valves, the cause is the post-combustion injection of fuel giving the 'boy racer crackle' from the exhaust. Even though we 'know' the cause, there is a prescribed test plan that has to be carried out with recording of findings - including examination of the valve gear, before the cylinder head is removed for visual examination. If this process isn't followed, the warranty claim will be rejected in it's entirety.
It's a pain as we 'know' the outcome but for the sake of a complete diagnosis, it has to be done - and of the thirty or so we've done this year, four have been due to other causes, so even if there is a common issue as in this case, it's not always the 'known' cause, even though the symptoms are identical.