Vaillant Turbo Max Problem?

Hi, I had a classic case of a diverter valve problem. No DHW - the plastic arm wasn't engaging the micro switch, but if I made the micro switch with a screwdriver, the CH warmed up, CH was running ok on its own. Replaced the diverter valve, rebuilt the boiler and it worked fine for a couple of days. Now the demand for either CH, or DHW is being recognised, but there is no ignition to the burner. I'm uncomfortable changing the circuit card as its £150+. Anyone got any other suggestions before I make the plunge for a new card? Could I have done something wrong with the diverter valve replacement? I've drained / refilled, bled, refilled, bled some more, fixed leaks and swore lots!

Thanks,

Doug.