A different challenge...Not an Xm...but of course, a non-runner.

[Dieselman]

Thanks. I fully understand this was my error, but have noticed a mix of metric and UNC on both engines, (iirc, 1985 and 1989). I didn't expect Jaguar to have gone from British Standard, to American Standard, before switching to Metric.
The Uk went Metric for engineering applications in 1965.

I find it hard to believe Jaguar couldn't afford a few new Metric threading tools. They must have been replacing tooling on a regular basis, just pick a change over date and go for it.
I guess Jaguar were just being stubborn, which unfortunately typified the British car industry and in particular British Leyland.