What could cause a clicking sound in an underfloor heating controller?
I called a number of Central Heating contractors and also Uponor, the company that makes the controller.

I was told that the clicking noise comes from the soft-fuses inside the controller trying to reset. They said box has failed and needs to be replaced by an electrician. Since the controller is the simplest model in their range, there are no diagnostic LEDs or other features that indicate a fault status, except the clicking noise of the soft-fuse.

The repair should only be about an hours labour plus cost of a new controller box.