Many suggest that the ideal tool is the one supplied by Jaguar in the toolkit that comes with the car.  This is a swivel wrench.  Note, however, that the spark plug wrench provided in the earlier cars won't even fit a spark plug!  Jaguar eventually discovered this problem and redesigned the plug wrench to actually fit a plug.  The difference is a hole in the top of the socket itself for the top of the plug to protrude through.  With the later wrench, when you put a spark plug in the socket you can peer through the U-joint and see the top of the plug.  Make sure you have the later wrench; the earlier one will cause nothing but consternation.  Of course, it may be possible to drill a hole in the earlier wrench to fix it.

Even with the correct tool, getting at the 1A plug is anything but easy. J Harper's solution is to modify the Jaguar swivel wrench so it fits better.  The following pictures illustrate his modification.

That last pic doesn't look like anything was changed.  "Yes, there was a point to that..... I only needed the rounded edge on the
bottom of the socket and I felt that if I ground down both sides I would weaken the tool too much.  I'm very accustomed to hanging wrench positions like 8 times to get a bolt to make one complete turn :-) (British engineering) so I felt that it would be better to refit the socket each time vs. weakening the tool.  Better to be slow and safe then fast and sorry.

"Also, I only use that wrench on the front plugs and use regular Sears stuff for the rest of them.  The reason.........not to put to much wear and tear on it.  That's not a high tempered tool so I save it for only the problem plugs."

An alternative plan is to forget about the Jaguar swivel wrench and use a standard spark plug socket.