occasionally, when the boiler ignites,the burners light and then go off, then on, then off again sometimes up to 5 times before igniting properly. Sometimes, very occasionally locks out. Anyone with any ideas on this fault.

John, May 2007

Sounds like it could be either a flue fault or flame recognition problem. Vaillant are a very robust boiler and they don't tend to suffer from trivial faults.
Either way it's going to be something involving gas or flue inside the burner chamber which requires corgi engineer. Sorry.

T.h.s, May 2007