Vokera Linear 24 timer fault?

The boiler works fine in both hot water and central heating modes until the timer is set. When the time is reached for the heating to start everything runs as normal until the selected temperature is achieved and the flames goes out. When the boiler tries to re-light the flame lights increases to max and then just go out. The boiler repeatedly does this until fault code 03 appears and from this point nothing happens. I've had heat exchangers replaced due to over pressuring problems, thermisters, gas valve, flame sensors, 3x PCB's and the selector dials at the front due to constant problems. I'm a little lost at what to do now, can anybody help?