Unfortunately on an eleven year old car some wear and tear is to be expected. Common problem areas on many modern diesel engines are clogged inlet manifolds causing a restriction in air flow, partially stuck open Exhaust Gas Recirculation valve causing excessive exhaust gasses into air intake on which the engine will not run very well or not at all, or a worn turbo or stuck turbo vane geometry.

The latter will cause an overboost condition which will trigger a fault code. This will cause the Engine Control Module to set a limp home mode which itself is not a fault. It's a protection against serious engine damage which will occur if the turbo boost is not controlled at source. If the turbo vanes are stuck you will find the problem will occur at around 3,000rpm at which point limp home will be set. Shutting the engine off and restarting will reset it, until you hit 3,000rpm again. So if that is your symptom then you can bet the turbo needs attention or replacement.

Then there's the Diesel Particulate Filter which on a car of this age will be well past its best. This could be heavily blocked and unable to clear properly with every regeneration. And that's assuming both you and its previous owners have driven it in the necessary way to keep it clear. Modern diesels are not suitable for local short trips. They need to be doing 'hot miles', that's mainly motorway and trunk route motoring. They're ideal for commercial use, but not for shopping and taking the children to school.

Many garages just don't have as good an understanding of modern cars as they should do. This is due to a lack of adequate training and investment, so it doesn't really come as much of a surprise that you're struggling to find someone to properly repair it. The truth is many simply don't know what the problem is.

I apologise for sounding critical but you probably should've gone for a petrol car rather than a diesel. Modern diesels give better fuel economy, and that's what attracts people to them. But they are also a LOT more problematic and inevitably much more expensive to maintain than a petrol engine car, so anything you save in using less fuel more than costs you in other ways.

No way do you put petrol in a diesel car . This can cause serious damage .

If neat, yes. Assuming a brimmed tank holds over 50 litres I wouldn't have thought a 4% petrol mix would hurt its injector apparatus. Don't they add petrol to diesel in the Arctic when it gets very cold? What is the mileage of this sick engine?