Halsted 102 Combi not firing up?

HI, moved in to a new house and although my Halsted boiler worked to start with, it now will not start the hw or ch. When i turn the hot water on it makes all the correct sounds as if it will start but then does not fire up ! i've had an engineer out who tried changing the pcb which didn't help (managed to return the new one thankfully as best part of £300) and change the thermistors which appareantly needed doing, but it will still not work. when it cuts out a green flashing light shows on the display. can anyone give any advice ?? thanks