likely the pump?

my grant boiler (riello rdb burner) was running fine for a few days after we got some more gasoil fuel, then it started locking out after every first run. now it won't work at all, it just locks out after the 12 second purge. the electrodes are sparking fine and the photocell works as does apparently the control box, it just seems like no fuel is getting to the electrodes. i did bleed the pump and it has plenty of fuel. cleaned the filter in the pump too to no avail.

i just don't know what could be wrong. i presume it is either the pump has failed, the solenoid isn't working or theres a blockage in the nozzle. however after the 12 sec purge there is a audible click when the electrodes spark up, so I'm guessing that means the solenoid on the pump is still working.

When i took the nozzle off to clean it there was fuel in the nozzle holder. i wasn't sure it would be a good idea to try and turn the thing on with the nozzle off in case it was the nozzle and i got a ten foot flame burn my eyebrows off or something so i don't really know for sure if the pump is giving pressure.

Just wondered if anyone had any suggestions, on if there's anything i can do to see where the problem lies before i start trial and error and buying components for it. thanks.