My 78 year old mother owned a Mazda 6 2.0 petrol 6 speed manual. The spark plug change intervals on this car are 62,500 miles or 5 years - I think. NGK plugs are expensive for this car so I bought Bosch plugs & replaced them. The car ran obviously better: eager to rev, no flat spot in the Rev range - a worthwhile change - sadly still a boring grey car.
Why in the world would Jag make something as simple as r/ring spark plugs so difficult? 
What about the face-lift V-12s (i.e. '92 MY like Superblack), particularly the 6.0s? I read something in the Jag literature once about supposedly the engine compartment/engine on those MYs was re-designed to make them a bit easier/roomier to (work on things), or some such. 
FWIW, the only thing I noticed different about the engine compartment in the 6.0s is the addition of that engine cover, which, although it keeps the valley clean(er), is probably a PITA to take off every time you want to work on something under it. 
That is how I do it on a lot of cars. It worked to "T when I swapped in new plugs on my daughter’s VW Passat. Hemi head. Plugs in a deep well
Same on the Predator, a Honda knock off on my old chipper shredded. An OHV engine. Old Troy bilt. I have two!!!
Not really a pita to remove that engine cover. 4 spring-loaded screws half-turn to lock (pozidrive head)
on a mercedes engine the entire thing is silicon aluminum,the heads the block all of it and with no steel liners.those things will easily get 300,000 to 500,000 miles out of them too.how they keep the piston bores from wearing amazes me the only thing i can think of is the oil technology . also a little off topic mercedes sl500’s just like old jags have had horrible resale values but i understand this is the year they start rising and start becoming collectibles. thats why i am buying an sl500 and just purchased an 89 VDP,between the 2 of them i may not only make some money but even better i get to drive these 2 fun automobiles till i sell them.