Westwood T1200 not turning over?

Yesterday, the mower was getting increasingly slower to start - was turning over but not firing. Thought perhaps it might be the battery so charged it. Today, reattached battery (definitely correctly), and the mower is not even turning over. Not even any clicking. Dead as a dodo! Also tried a different battery, which did the trick last time, but no difference. Any idea what might be broken? The solenoid? Starter motor? Explanations for technical idiots please :-) Thank you!