UK Car Repair: Tractors will not start!!, skilled mechanic, jumper cable

Question
I am still having the same problem with my B414. It will not start. I bought a new battery and solenoid and cleaned all connections. When I turn the key the solenoid makes a noise (buzzing or clicking but no turning over.I took off the cranking motor to see if it was stuck but every thing seems fine. What could be wrong?
The solenoid has four posts. There are two large posts for the power from the battery and one that goes to the cranking motor. There are two smaller posts. One is for the ground which is taped together with the power wire (same as all vehicles). The next post has a connection that I am not sure where it goes. This is not connected to anything. Could this be the problem?
Any advice would be greatly appreciated.
Thanks in advance.

Answer
I don't recall this question!  None of the connections on the solenoid go to ground. There may be a separate ground lead but it will be connected to the outside case. The large ones feed the starter, one small one comes from the ignition relay or switch, this operates the internal solenoid which then connects the large posts. The other small feed is usually a spare positive feed which can be used for auxillary functions. (This is worth investigating). As you say the solenoid "buzzes", the feed from the ignition seems to be good.  The problem is likely the starter OR the Engine!  Make sure the engine is free to turn by hand.  A skilled mechanic can by pass the two large terminals with a switch connected in series with a large jumper cable.  The cable is connected to the large posts and the switch activated momentarily to see if the starter actually turns.  Warning!  If you do this make sure the vehicle is in Nuetral and the Handbrake is on.  The ignition must not be on!