I am unfamiliar with the v12 engine but I do not think the engine needs to be removed to remove the head(s).
You might not need to replace either of the heads, I am sorry if I sounded like you had to. If the head did infact warp then a machine shop can usually machine it out if it is not an extreme case. If the head cracked then you would probably want to find a replacement. You should not need to replace both heads if only one were to be bad.
However I would not assume I had a warped head, it is just something that can happen due to high heat. I would guess it is not your problem. My guess is you have one or more burned valves or valve seats.
However it could just be that some of the valves are out of adjustment.
To check clearances you need to first find the firing order of your engine. (jag-lovers probably has a guide for this as well.) Then you will set the number one cylinder to TDC.
After that you can take the valve covers off to expose the cam shafts. [You should replace the gaskets for the valve cover upon reinstallation or you may leak oil]. There is one camshaft per side. You will notice the camshaft lobes. There will be 2 lobes for each cylinder.
One of the lobes opens the intake valve and one of them the exhaust. You will notice that the lobes on the number 1 cylinder at TDC will both have their elongated parts sticking outward opposite of eachother. In other words neither lobe is pressing on the valve tappet so both valves are closed. You will use a feeler guage to measure the distance between the camshaft lobe and the tappet while that cylinder is at TDC. Then you will rotate the engine(by hand) to bring cylinder number 2 to TDC and you will then measure its lobes with the feeler guage. Continue for all 12...
If anything looked scored or otherwise deformed that is a sign of a problem.
Although I do not think there is a Haynes manual for the v12 model I recomend you pick up the Haynes for the xj40 anyways before you do any real work. You can gain a lot of the technical ideas from the book and then apply them to your car.
Injectors can be listened to with a mechanics stethoscope or by putting a bar up against the injector and listening to the bar. The idea is to hear the injector clicking as it opens and closes to inject fuel. The bar will help amplify the sound comming from each injector. You should hear a fairly fast paced ticking if the injector is operating.
If the plugs were black and gummed up then there may be oil entering the cylinder. Is there any noted oil loss? What do the new plugs look like?
Did you do a compression test? This is an easy test. If you have good compression then your problem may not be in the valvetrain or head at all.
It might be worth your wile and time to take it to a shop and have them look at it. They will be able to perform a leak down test quite quickly and note other signs that will point to the probable cause as well.
Again I don't know what the specific numbers are for valve clearance, compression pressures, etc. However they can probably be found here without much difficulty. ImageShack® - Online Photo and Video Hosting
is a photo of the camshaft assembly for the xj40 v12 to help you get an idea.