Baxi 310 fi water heater cycling continuously?

My Baxi 310 fi water heater sounds like it is cycling continuously as though it's trying to start/light. It makes a clicking noise and sounds like it tries to surge on, then clicks as the surging dies back down, and then repeats every few seconds. The green power light is on, as is the central heating light - no faults are flashing. Any idea what could be causing this? It doesn't seem to be heating the water properly either.