Ford fiesta 1.4 tdci zetec "Chugging"

Hi all, I've come to ask for your knowledge and wisdom in helping me fix the issue with my car.

I have a 57 plate Ford Fiesta 1.4 tdci zetec which has only just past 18k miles and it all started with it occasionally "chugging" when accelerating and the revs reaching around 2k rpm. It began happening more and more frequently and I noticed it tended to happen when the engine was under more stress (going uphill, even slightly). It goes into the "limp home" mode which is a pain. I am able to reset it but just find myself waiting for it to happen again whilst driving. Checked the DTC codes under the on board "test" mode but it shows no codes.

I have taken the car to a garage and had 2 of the injectors changed as they said they were faulty, along with the fuel filter, oil filter and oil change but no joy. It now goes straight into "limp home" mode around 10 seconds after starting the car, which idles and "chugs" then enters this safe mode.

I'm now over £1200 down from garage costs and at a loss what to do next. I have searched for hours but haven't found anything conclusive.

Any advice will be greatly appreciated.