The earth straps are connected fine but your right in that it is a bad earth somewhere, the potential difference between the engine and neg side of the battery is 5 volts which is quite a lot considering it should be 0... Ran another earth strap and it runs fine now...Check the earth strap from battery -> chassis -> engine/gearbox. If you have a multimeter switch it to volts and put one lead on the battery negative terminal the other on the engine/gearbox.
It should read 0 volts, any reading in the meter is the potential difference (volts dropped over the earth strap) and therefore the number of volts not reaching the starter motor.
All the best
Or it did till it spat all its coolant out on the M65 yesterday...