Yes, connect upto the wire that goes to the alternator, I'm not sure if you'd have full access to do this with the alternator in place, but you can try it (Just make sure the battery in the boot is disconnected) - The alternator is connected directly to the starter.
It's all trial and error, we will find out why your car won't start.
EDIT: And just to clarify, EVERY single time that the car doesn't start, does the starter make a clicking noise ?
If so, that suggests the wiring from the ignition to the starter is fine... It suggests a power problem from the battery.
You've already added another earth which didn't help. So it must only be the power getting to the starter.