2015 Vauxhall Insignia 1.9 cdti 120. Poor running

[Lee21]

Any ideas . . . . ?

Current mileage: 161,000. All motorway driving.

Regularly serviced. Air & fuel filters.

Sept / Oct / Nov . . . . Engine started intermittently flat spotting. No EML light. No fault codes stored.
Ran injector cleaner through x 2 over course of 8 weeks. No difference.

EGR valve replaced December @ 158,000 miles. No more flat spotting but engine not running
quite as well as it used to. Not pulling as well and lower MPG. Very minor intermittent hesitating as driving.

Engine sounds ''throaty'' and harsher on overrun. Engine feels restricted and not at 100%.

Please note:
Car will not rev over 3000 rpm on idle intermittently but then you can red line it sometimes.
Fuel tank has been run very low on a number of occasions in the past
Wynns Formula Gold Diesel treatment used 800 miles ago. Not difference.
Occasional burning smell coming into cabin from engine bay.