When you say juddering, I take it that the engine is either missfiring or running rough?

Given what you have already done and that you want to reduce the expenditure, I would suggest that you look at the least expensive areas:

1.Check your air filter to make sure that it is not clogged up.
2.Check for split vacuum hoses/pipes.
3.Check for split intercooler turbo hoses
4.Check the cam/crank sensor to see if its loose/cracked or damaged, if unsure

It might be worth replacing (not too expensive)

You don't say how many miles your car has done, but would also suggest that you check around the injectors, to see if there is a chuffing sound when the engine is running, also if "black tar" can be seen around the injectors. If so you may have injector "blow by" which is when the injector does not seal against the injector seat in the cylinder head.If this is so then you will need to get it sorted asap.