How can I fix my Vaillant Turbomax 242E heating boiler?

My faithful Vaillant is now 10yrs old with a couple of parts changed over the years, divertor Vv and clock if my memory serves me. On return from my hols recently the heating came on for a short while and then went off. Burner lockout comes on after the 10s ignition time passes. My gas engineer pal has checked everything again and again having replaced the main switchboard to no avail. This was changed as the gas solenoid was only getting about 19Vdc and according to the book this should be 200Vdc. The flow charts also seemed to lead to change the main switchboard. Can anyone point us in the right direction?