Juddering Acelleration After Injector Seal Replacement

First time post so hello, I will get round to introducing myself at some point but for now I have yet another concern about my focus tdci.

Some background, I bought the car a month ago, my first car after owning bikes for a couple of years, it's a 55 plate 1.6 TDCI Focus. When I bought it I was told (with paperwork) that it had had a new turbo and oil change. Within a couple of weeks the turbo went so I had that done (£800) then last week the seal on injector 3 started chuffing so I had that done, since I had that injector replaced I have now noticed a juddering under acelleration at certain revs.

I've googled it, which led me here in the first place, but as always there is no consensus but it's worth a try...

Essentially if I accelerate from a stop up to 3rd gear then change into 4th around 30-35mph (1.5k RPM roughly) and then try to acellerate gently the car starts juddering, if I keep the acceleration going the juddering gets worse and worse and get's fairly violent. I can feel it through the seat and the steering wheel, but not through the clutch pedal. once acceleration levels out the juddering stops and if I take my foot of the pedal then re-apply and accelerate the juddering is mostly gone or completely gone.

Any thoughts?

One thought is that when they did the injector seal something was amiss and there is a minor blockage on that cylinder. The juddering is pretty similar to the frequency of the chuffing. Google keeps bringing up scary words like flywheel and clutch... If it's something to do with those then I'll be setting the car on fire...

I am tempted to go back to the garage but I want to know roughly what I am talking about if/when I do. I am also tempted to have a bash at replacing the fuel filter, seems relatively straight forward and might improve fuel supply which could help *if* it's a fuelling problem.

Any leads or info would be greatly appreciated!