Sporadic Failure to Fire?

My Potterton Netaheat 10-16 Mk II F generally works very well, However, during the last year it has occassionally failed to fire up. It will begin the start-up procedure and when it gets to the stage where the pilot should light, the pilot fails to light. There is gas, water and electricity and the pump is fully functional. A few days later, everything is back to normal. What should I do, please?