Hello Dave and Welcome -
My name is***** happy to help / advise you on this..
Certainly it does look like - and it would be a good bet , that its dirty injectors. What happens is espeically with that sort of mileage, the pintles on the injectors get sticky and effect the spray pattern which can be enough to cause what you describe - espeically of all injectors are affected..., So yes, I would suggest go for a good fuel injector / system cleaner - I can recommend the use of Forte products Dave.. These are not sold in shops - they are a trade only product - for just these sorts of issues... but you can get them from various sources -one of which is HERE
.. This Forte product is not the cheapest but its the best! Given the engine / system you have - I would suggest to buy / use two bottles - and at the same time - fit a new filer - pouring half a bottle into the filter when fitting...and the rest in the tank - so that it works as you drive.
Give it enough mileage to have an effect / work Dave - it does work very well so if its that issue, it should work for you. If the injectors are really bad though, then they would need to be removed and cleaned at a diesel specialist - ultrasonic cleaning will often resurrect a bad injector / save renewal.
So its certainly worth doing that Dave - its the cheapest way forward and has every chance of working -its worth a go, as I say, its made for just these sorts of issues.
Again - IF it doesn't cure it - I would then suggest have a diesel specialist as opposed to a dealer check it for you - they have all the test equipment needed and if often a better option - and cheaper, than dealers. Plus you don't mention any fault lights... with the absence of such, dealers tend to be somewhat foxed - whereas a specialist as above will test the systems fully / properly to find the fault...be it injectors / pump issues etc.
So thats the way to work forward Dave and get to the bottom of this for you -
If there is anything more you need on this or would like to discuss - by all means
get back to me and we can carry on... >> Meantime >>>>
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Kind Rgds, MIKE.