Thanks to contributors.
It's been a long time in the fixing.
Eventually got the permanent DPF fix done, at some cost.
Subsequently discovered the doughnut hoses were split, which would have caused my symptoms and was an easy, cheap fix, £7.60 each for 4x heavy wall silicone replacements.
We live and learn.
I read many things on various topics saying go for HP smoke test first. Certainly wish I had now.

Nearly every time a restricted performance or a DPF issue are caused by boost leak.
Glad you got it sorted

. Cheers, Greg
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2011 Vogue SE 4.4 with lots of toys in Stornaway

DPF issues usually an intake leak cause mainly. Shame you didn’t do a smoke test first
Glad it’s sorted though  Gone to a good home: 2011 4.4 TDV8 Vogue SE Buckingham Blue with Ivory and clear glass = "Rory"
2025MY Defender D350 90 in Silicon Silver on coils
1974 Series 3 Lightweight = "Millie"
Many, many other Landies over the years
My preferred specialist: www.glenrands.co.uk
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Yeah. You know that bright red doughball sensation you get when you didn't do what smart people suggested all along? Well, yeah.

Kot wrote:

Would love to get my hands on a flow chart regarding the algorithm

Not quite a flowchart, but I summarise as below

Click image to enlarge

 “If it ain’t broke, don’t fix it” -where’s the fun in that?

In short, it’s guesswork!  “If it ain’t broke, don’t fix it” -where’s the fun in that?