I have a Baxi Solo 2 PH60 boiler which has an intermittant start up problem?

My boiler was installed by British Gas in 1998. It has been fine but for the last couple of months it has developed an intermittant fault. Although the boiler on light is lit that is where it stays ie the fan light, pilot light and burner do not sequence up. If a reset a number of times eventually it will fire up. Any advice gratefully accepted.