Thanks Ex Brummie.
I started out by disconnecting the joint in the fuel line upstream of the heater control, flame failure valve and remote fire valve, which were all connected directly to each other in one line. Fuel came out of the pipe but no fuel came out of the exit from the heater control. I was unable to ascertain where the blockage was. I have now removed the heater control, flame failure valve and remote valve and disconnected them from each other. There was a tremendous amount of debris in all the units so I suspect that the installation has been unused for many years. I have now thoroughly cleaned all components and there appears to be a clear rout through each component and I hope when reconnected to the boiler and oil supply line that this will have resolved the problem. I have also discovered on cleaning the heater control that the threaded hole which was the subject of my initial question has the words 'Over Flow' hard stamped into the body casting. There is no latch on this model of heater control.
Thanks again
Derek