My Valiant boiler pilot light won't light?

You need to be a bit more specific with the model of Vaillant that you have, but from what you are saying, see if you can see the spark, if you can't - Then you may need new HT Leads and Electrodes... If you can clearly see a spark, make sure the gas is on... Then it could possibly be a PCB or Gas Valve but it's really impossible to say without knowing exactly what boiler it is and without being there.

GasAid.co.uk
December 2006