I'm in exactly the same position. Inherited a cooker with the house. I don't remember if I ever saw the clock on it working, but it doesn't now. The buzzing/ticking noise is coming from my clock/timer unit, which seems to indicate a loose connection. The part seems to be no longer available (Invensys el2000 819.021 3-button timer), but alternatives would appear to cost about £50!!
I'm still trying to see how I can get the timer unit out of the cooker without having to take the whole thing apart.
Sue P, December 2010