Dumper fixed (for the time being)
Turned out the poor performance didn’t have anything to do with the engine – the right hand front brake was seized on! Good job I didn’t go to the hassle of changing the diesel injector!
The brakes have now been sorted together with new oil, new oil filter, air filter and fuel filter. I am proud to say that I managed to do it all myself. It seems to run a bit better although I do wonder if it was worth all that effort.
Despite its age, parts are readily available and I even managed to find a maintenance manual online.