Netaheat failure to ignite?

Netaheat 10/16 Mk 2 has developed a fault.
When the boiler is activated either manually or by the timer the fan starts and after a few seconds there is a click. Presumably some sort of pressure switch responding to the pressure inside the boiler. But thats it .... No repeat clicking noise as the pilot tries to light and therefore no boiler ignition.
Any suggestions would be greatly appreciated.
Many thanks