This morning the gas bolier fired up as normal but after about two hours we heard what is best described as a clicking sound from the boiler thermostat & the boiler would not fire up. I turned the boiler thermostat off & left it for a while & then turned up the boiler thermostat to its previous level & the boiler fired up & operated as normal. The boiler is working OK this evening. The clicking sound was as if the boiler was trying to switch itself on but could not. Any ideas please?