My potterton netaheat 10-16 boiler stopped working!?

When the room thermostat gets below temp, the boiler kicks in (pilot light and fan) and makes all the right noises but it does not seem to light up to heat the water. On a few occasions it does for just for a few short mins but then stops (fan keeps going and pilot light stays lit) Checked room theremostats etc
It's had a new relay but no joy!