Boiler fired but cuts off (Glowworm Micron 60FF)?

My boiler has suddenly stopped working. When it is reset, it starts the fan, and begins to spark presumably (as I hear loud clicking sound). The casing warms up but the unit cuts out in about two minutes. Sometimes it attempts to restart the sequence again but sometimes it doesn't. The Reset light turns solid green when the unit stops. No hot water and central heating. Manual says it is a Non-Volatile Lockout. (What does it even mean?)

Any help will be grateful.