Garage has kept car for over a week hoping that it would fail to start so that they might have a chance of finding something wrong - but it has not failed to start even once! Neither I nor they can get a single explanation from any quarter of what it could actually mean if the yellow key warning light does not come on. There are plenty of explanations for when the light comes on and does not go off but not if it fails to come on in the first place.
All of the suggestions from other readers so far are much appreciated and my garage, who are very conscientious engineers but with not much experience of Alfas, has applied them and checked all the connections, earths etc as suggested and all were found to be good and clean. Battery has also been replaced. The problem now is the unpredictability of starting and not having a clue as to what to do about it or what it could mean when the key light does not appear - surely somebody somewhere knows what it means when this happens? The car runs beautifully except when it doesn't start.