Had my injectors out today there was no issue, I just wanted to replace the seals and washers before something does happen.
I didnt get much history with my disco however the injectors were pretty spotless. Since owning it iv fired in shell/bp filtered diesel and millers carbon cleaner from halfords as there was already a few bottles in the disco when i bought it.
Im assuming this has helped keep the injectors in this condition. Worth a try!
Also read a forum where after using additives and better quality diesel there was a slight improvement in mpg.
I also have the egr bypassed.