The engine is normal running temperature,
Accelerating in higher gears 3, 4 or 5,
Glow plug light illuminates,
Engine dies and cuts out.
Depress clutch, maintain motion, switch off ignition, immediately turn on ignition to restart engine, lift clutch and proceed as normal.
Anyone know what sort of problem i could be having?
Seems like the car is going into Limp mode or something along those lines.

 

While I have no answer for you, I have a few questions, which might give us extra info for someone else to have an answer:
I was struggling a lot with "Glow plug light illuminates," and what this may want to tell us...

My long train of thinking was: glow plugs are used in diesel engines. I know nothing of diesel engines. I wondered, if those glow plugs have lights incorporated or if you are talking about probes. which you hold onto the glow plugs (or spark plug, if you have used the wrong expressing) - and reading further I wondered how you would possibly see those lights while driving. All very confusing. Until I finally figured that you most probably refer to the light on the dashboard, which may look like a glow plug.

Thus, here, more than in another thread, it would be important for you to give us the information WHAT kind of X-Type you have. Is it now a diesel or not.
Other than that: Model Year? Engine size? I figured already that you seem to have a manual transmission...

And if you get the engine to operation temp, it seems to be the case that the engine is most running, and then occasionally it cuts out? Is that what it is?

That would sound e.g. like a short circuit, an intermittent short circuit. Is there a proper ground connection? I could tell you that one important ground connection is on top of the automatic transmission - which however you don't seem to have, and I have never seen a manual.

Have you connected a OBDII scanner? That's always the first step. Without that you could just as well hold your finger in the wind and have a guess, what's wrong with your car...
Those scanners cost meanwhile less than AUD$20 on Aliexpress or Temu.

PS: I just quizzed the jaguarforums-site, to get the info, which you did not give - so you have a X type 2.0L Diesel Manual.maybe that helps someone in giving an answer...

PS2: Wikipedia tells me that your 2.0 diesel is a 2.0 L ZSD-420 I4 (turbocharged diesel)
This tells me that you have one more possible failure source: A turbocharger! (Of which I know nothing other than that the term turbocharger normally refers to chargers, which are
exhaust-gas-driven and operate on 150.000-200.000 RPM (that's up to 3.333 rotations per second!) and makes me wonder, how it could have lasted for all those years without
spewing broken parts into the air-intake killing off the engine...)

 

When you say the engine dies does it stop? Or does it lose power but continue running? Limp mode would cause the engine to lose power but not cause the engine to shut down altogether.

I second Peter's advice, put a code reader on there. If the check engine light is not yet illuminated look at pending codes. The fact the glow plug light comes on just before the engine dies suggests an electrical rather than a fuel problem. Probably.

 

If the "engine "cuts out", I took that as "engine stops completely". I also did not understand the addition of "limp mode"...

But I am writing, as I had one more idea: It something electrical stop randomly all of a sudden, it could also point to cold welds (=bad/cracked points of solder) in a module.