Spark plugs are an easy do-it-yourself project on an XJ6 but can be a bit tricky on an XJ12. See the Jaguar Forums to learn about changing V12 spark plugs. Six cylinder spark plugs are easily accessible.
Most Jag owners prefer the orignal Champion spark plugs. Other types are acceptable and some owners like platinum plugs. There's hours of discussion in the Jaguar Forums about everyone's favorites.
For 6 cylinder cars use Champion N12YC (merchandising number 404). The NGK number is BP5ES (or GR4), and the Bosch number is WR8DC. Gap in all instances is .035". I am not sure what sparkplugs are correct for the V12 engines so I'll remain silent on the topic.
Change your spark plugs on a cold motor and always use a dab of anti-seize compound on the threads.
Sparkplugs should last about 30,000 miles (48,000 km) under ideal conditions. Oil burning will decrease the life span as will a too-rich fuel mixture.
Inspect your plugs on removal. They should be light grey or tan. Black, fluffy plugs indicate a mixture problem (too rich) as do near-white plugs (too lean). Black, oily plugs indicate oil in the combustion chambers, from worn rings and such.
The distributor cap and rotor should also be changed every 30,000 miles or whenever the contacts show obvious corrosion or burning. Inspect them every 12,000 miles.
Spark plugs wires are a common mis-fire gremlin and they often fail intermittantly, making it difficult to pinpoint the fault. Plug wires ahould be replaced every 60,000 miles (96,000 km) on general principle, more often if inspection reveals burn spots from "leaks", cracking, or abrading. High under-bonnet temperatures take quite a toll on these wires. One way to check for "leakage" is to run the engine in a dark garage (don't asphyxiate !) and, if they are leaking, you'll see a real light show under the hood. Of course, this isn't the final word on spark plug wire testing but it can be fun.
Regularly check the primary igntion wiring to the coil and ballast resistor. This wiring, due to it's location, is very prone to heat deterioration and often frays and splits.