Clutch & low gear cruising.

Goudy

Ok, EGR is blanked so the air intake side appears fine.

Now what's left, fueling and fuel timing.
I would suspect you would get a DTC or a limp if the powertrain ECU had issues with a crank or cam sensor, though I wouldn't rule them out, can you read the ECU with a scanner?

Fuel pressure might be an issue.
As the rpms reduce there might be something causing the rail pressure to drop below required spec so cuts the injectors.

As I wrote earlier, take you foot of the pedal on the over run and they inject no fuel, all that fuel in the rail bypasses the injectors and returns to the pump or filter depending on fuel temp.
If there is a problem with rail pressure, dipping the clutch as you slow or slowing in a high gear results in low rpm and low pump speed and there just might not be enough to keep the injectors firing.

I've only ever suffered at the other end, high load high revs would trip the my 2.0 TDCi into limp and give me a fuel metering valve DTC, the metering valve just couldn't open enough to meet demand.
I ran a leak off test that showed a couple of injectors had excess leak off which reduced the rail pressure when demand was at it's highest.

What's the mileage, and what mileage has it done on those injectors?

It's a long time ago since I had one, but I remember these being very touchy on fuel pressure, so much so that they would stumble at start up if the crank speed was a little low due to a lowish battery, it felt like they were running on two cylinders and you couldn't coax the throttle up to get it to run smooth. The trick was to hold the key over on the starter motor a fraction of a second longer after it tried to fire.

It was common knowledge they didn't like certain fuel filters, they liked genuine or OEM (Delphi, though Delphi did a budget filter they also didn't like, think it had a E at the end of it's product code).

It's worth checking for fuel leaks around the fuel filter, rail and injectors and their unions, also see if there is any evidence of fuel leaks under the engine (rad side).

A new fuel filter is worth a try and is cheap enough (buy a quality one), but you have to prefill them with fuel before fittiing otherwise you'll air lock the fuel system.

B0bbyEwing

Few quick things as I'm at work:

Aware of the fuel filter situation. I bought Delphi but not the E variant. The filter was also changed in August, the previous had been in 2 years.

Had a leak off test done last Xmas '21 for a different issue. Turned out to be glow plugs but leak off test reported no problem. Granted, 11 months have since passed so who knows. 

Have access to Forscan although not premium version. Can read DTCs but don't know what I'm doing regards reading any specific sensors, plus I think that may be paid-for territory. 

The temptation to replace the car is strong, but overall its working so I feel I'd be wasting money & the best action is maybe just running it until it dies. 

Goudy

Things can get expensive with the fueling system.
Injectors are nearly £200 a pop and that's before fitting and coding.

I seem to think the injectors last somewhere between 80k and 120k before they give trouble, though a lot of owners gave up on them long before that as they did tend to cause a lot of other headaches.

I am not sure how true it is but there were rumours that around 80k the injectors needed recalibrating and the new values recoding to the powertrain ECU, though that might have been a Ford short term, cheap way out of warranty claims answer to some inherant fueling problems.

Back in the day the Duratorq name didn't take long to morph into Suradorq.

It's worth checking if you can see anything obvious, like leaks around the fuel system, test the tightness of the fuel unions between the rail and hardlines and injectors, then try scanning for any codes, your basic scanner should pick up any generic codes.

As it's a Euro 4 diesel, you would think it's probably coming to an enforced end soon enough anyway.

Something else just popped into my head and I mentioned it before.
Ford revised the Cam sensor on these, the original was black plastic and iffy, the newer one grey.
They can cause misfires and cutting out.
If it's the original black one, it might be worth risking a few quid and replacing it.

B0bbyEwing

Is that sensor easily visible? 

And sorry I forgot to add the mileage. I think it's about 156k now. 
As for how old the injectors are - who knows. Older than 2 years is all I can honestly say. 

Goudy

The cam sensor is in the cylinder head, tuck up under the right end of the fuel rail.

https://www.youtube.com/watch?v=8hSra6WbrTw

I'm not saying that's your problem, just another suspect.