How can I fix my central heating timer?

My Sunvic Select Heating Timer was making a clicking sound every half second or so, but the heating was still working. I turned the timer off one evening, turned it back on in the morning, the clicking was worse, and the heating had stopped working. The red light still comes on as if it is switched on, but no heat!! Can anyone offer any suggestions as to what the problem might be? Any help is greatly appreciated!