Firstly by draining the fuel from the tank via the hose to the filter and discarding the old fuel. Then emptying the fuel from the primary and secondary fuel filter and replacing. Next we put fresh fuel into the tank and filled the filters and started bleeding the system using the lift pump and followed the instructions given in the workshop manual (thank you to the people responsible for those on this site too.) Unfortunately at the first visit we could not get fuel going to the injectors so we had to leave the job unfinished. The pump was suspected. We got various information from internet searches, schematic diagrams, seal kits but ultimately none were required.
At the second visit he took away the injector pump by disconnecting the pipes to the injectors, the throttle cable and the stop lever and scratched two marks on the top to align for the replacement. This pump was taken to a place for testing and after a day it was working again. Something inside had stuck and was freed. It wasn't as bad as at first feared since the machine always used to start easily without hitches.
On the third visit the injector pump was replaced and the system was bled and after an hour of messing about with various things everything was tightened up and fuel appeared at the end of the pipes. It wasn't long after that we tried it and at first nothing happened. So we bled the pump some more and got more air out especially using the pair of bleed screws.
Finally after another few minutes of patiently bleeding the system it fired up. We hope to clear the area and move it for the first time in quite a while later this week.
Satisfying to know that somebody else will now be able to move things about and use a classic machine that has been preserved for quite a while now without any work to do. JCB 3C III
looks very tidy too.