I had this very same problem with my 2006 1.9tdi golf.
Took the car in for a service when I first bought it, mechanic said the glow plug in 4th cylinder kept spinning, so they had to drill it out and rethread the glow plug port.
That's when the problem started.
Car started juddering under idle, but seemed okay when actually driving.
Got the car diagnosed and turned out to be a "Cylinder 4 missfire".
I used diesel injector cleaner from Halfords, solved the problem for 2 weeks, but it came back and it looked worse because the car was smoking like a steam train.
Changed the £70 loom, which cost £60 labour... didn't work.
Then got a refurbed injector for £250, plus another £60 labour to change it and it sorted the issue.
I tried to change the injector myself, but I didn't have the tool to remove the last bolt from the EGR valve.
I used diesel specialists in Isleworth, London.
But don't know how good that would be for you due to distance etc.
None of that was helpful, but apparently if you check the loom and it looks tired due to heat etc.
Then that could very well be your problem.