Based on all of the things that you have swapped I would say that the problem is in the wiring harness for the starter circuit-the high current side from the battery through the solenoid to the starter. You can use a voltmeter to check each section of the circuit by connecting the leads on each end of the circuit you want to test(i.e.-from the + terminal on the battery and the post on the relay). With a charged battery and the meter set on dc voltage it will read 0v when you have the key on and it should read some voltage(ideally less than 1v) while you are cranking the engine. Repeat this test on each part of the circuit(wire from + on battery, solenoid and wire to starter) and see what you get.

The idea is that if the part of the circuit you are checking has very low resistance then there will be very little voltage registering on the meter while you crank it and if there is alot of resistance then you will see a larger voltage on the meter. Basic principle is that the battery will supply 12v at its terminals and any resistance in the circuit will decrease the available voltage AT the starter. You WILL find the problem if you follow this simple test....
Another simple test on this circuit is that if a component(cable, connector,relay) has high resistance then that part will get hot when you are cranking the engine. The part is converting electrical energy into heat-this is also referred to as the 'smoke test'...

 

Thanks very much for the reply I'm not at home at the moment but I will carry this test out tomorrow and let you know how it goes

 

For anyone wondering it turns out it was the starter gear! Electrical system all turned out fine.

Cheers