Clicking could be the starting relay near the battery or the starter motor solenoid - which is it?
If the solenoid is clicking and the battery is in good condition, then it generally signals a poor high current supply from the + and - battery terminals. Or a stuffed starter motor - sticking solenoid, burnt contacts etc.
The major engine earth I am aware of is at the mounting bolt at the front of the engine block for the cold water pipe (Stainless steel U shaped mounting bracket) from the radiator lower hose. Give that a clean and retighten. The bolt is holding down about three items - bracket, earth lug, washer - all into the alloy block. Check the mounting torque.
If you have some jumper leads, use one from a good mounting point on the engine block to the negative terminal of the battery, that should give you a good earth circuit for testing.
On the positive terminal check that the battery post terminal and the sub plate bolted to the terminal is clean and secure.
If you can get the engine to crank with the voltmeter across the battery terminals, if it drops below 10 volts, it is an indication that the battery has a low charge or getting sick. This is a difficult test with a DMM as the scan rate is usually too slow to give an actual reading - unless you have a capture model. A moving coil meter is a better tool for this test.
If you are handy with a DMM then run your headlights on high beam and measure the voltage drop across each +12v joint and all negative earth bonds. If you see significant variations across the earth bonds then they could be heating up under starting current, making better contact - enough to get a good cranking voltage, then cooling down. When you do see a significant variation, run a bypass cable across the joint to see if the voltage drops. This way you can possibly localize the problem earth bond or +12V terminal problems etc.
