Help !!!

I am having problems with my Vaillant TURBOmax VUW 282E combi , for 1
week now since we returned from holiday we have had no heating &
no hot water.The heating was left on whilst we were away , but
appears to have locked out sometime during the week.
The boiler will not light , just keeps locking out.I have changed the
main switchboard and the main gas solenoid.It goes through it's
sequence , the pressure differential switch is working , it sparks ,
but does not light.There is gas before the gas valve but not after
it.
Anybody any ideas , your suggestions would be greatly appreciated.
Thanks