Danesmoor 15/19 locking out after having pump and everything else changed c?

Is there by any remote miracle anyone out there who could shed light on a nightmare of a problem?
We have the above model boiler which has served us well for 11 years until one day it stopped working, suddenly. We have now had 5 heating engineers out who between them have changed the pump , twice, also every moving part, several times... replaced all pipework apart from the pipe leading from the tank which is underground to the connection on the wall outside the house. The current engineer has rigged up a bucket filled with oil using that as a substitute tank and it worked. Resulting in now having an oil lifter on the outside wall of the house which looks dreadful to say the least, though now it does actually work but will have to be removed because we live in a listed property. If by any chance anyone can shed any light we would be very grateful by the way we have none return valves fitted everywhere.