Does it try to light can you hear it spark?
If it is intermittent the low gas pressure has a habit of moving too low causing the boiler to cut out a simply adjustment can fix this. If it is sparking but not lighting if there is gas at the appliance it could be the gas valve or the lead too it which contains a bridge converter that can burn out . you can usually remove this lead one screw and have a smell in the ports for electrical burning. Remember to turn off the power though !
Gastech, June 2011
