NIGHTMARE 2011 zafira cdti 1.7 injector 4 misfire fault

Hi all, hoping someone can help me out here.

I have a 2011 zafira 1.7 cdti, car has been perfect for the last 4 years then out of the blue it started juddering, limp mode and smoke out the exhaust.

I scanned it with my snap on solus and had an injector 4 code misfire so I ordered a new one, along with a new bosch fuel filter, changed it along with new seals and coded it.

Car now starts but have a code telling me small quantity not learnt, so I go into special functions and start the test..... it won't complete it, car stalls and won't restart, now have a dpf fault, removed dpf internals and map out, car starts again but still won't learn the small injection quantity, its up to temp, doors closed, everything off etc and now the injector 4 misfire and limp mode are back.

On searches seems a common fault with the 1.7cdti