Jim,
Stepwise,
1) Using voltmeter, measure the voltage across the battery posts while cranking, if it drops below 10 volts it is likely the battery.
2) Using voltmeter, measure from the battery ground post to frame. If it shows significant voltage while cranking it is the battery ground cable.
3) Using voltmeter, measure from the engine block (unpainted metal) to the frame. If it shows significant voltage while cranking it is the engine ground cable.
4) Using voltmeter, measure relative to the chassis each connection in the postive path on through to the starter (sequentially) looking for a substantial drop in voltage below 10 volts.
The fact that it is clicking indicates that the solenoid is engaging, enabling the starter motor, which then consumes enough current to cause the postive voltage shared by the starter motor and solenoid to drop enough to cause the solenoid to drop out. That releases the solenoid, the starter motor no longer draws current, then the voltage rises enough to engage the solenoid again. Faster clicks tend to be bad connections, slower clicks tend to be bad battery.
Then again, it could just be Lucas...
(Wilf - I apologize most unreservedly).