Roomster won't go over 3000rpm

Hi all

I have a Skoda Roomster 2010 1.5 Diesel. Just bought it.

Guy said it had 2 injectors and the car failed straight away and I had another injector changed.

Garage won't diagnose anymore but the car won't make more that 3000rpm.

I've diagnosed several things it could be and replaced the fuel filter but still not sure what it might be.

Great little car, don't want to give up on it.

Wonder if anyone has any ideas?

Thanks, Justin