Do you mean the solder is failing or is the copper track pealing off the board? If it is the solder failing it is generally a bad technique in soldering but practice makes perfect. Remember flux is a technicians best friend. Although most solder has a flux core it is not enough. Invest in some liquid flux. If your problem is the copper pealing of the board then I suggest you bond everything to the board with epoxy.

 

The solder is coming away from the pad as the pad is quite small and I'm using 3amp diodes so there's not a lot of room.

Is puttiong the diodes 3 - 4 inches away on terminal block really a poor idea?

 

The wire between the coil and the diode will have some inductance, but relatively small (around 40nH-100nH) compared to the large inductance of the solenoid. It should work OK. If you want the current through the solenoid coil to decay more quickly, you can use a Zener diode rated for 12v-15v back-to-back with a standard diode. The Zener cathode should be connected to the MOSFET drain/solenoid coil junction.

 

The solder is coming away from the pad as the pad is quite small and I'm using 3amp diodes so there's not a lot of room.

Is puttiong the diodes 3 - 4 inches away on terminal block really a poor idea?

There is no reason why you can't do this. Closer the better even if it is just for neatness. Try resoldering and a dob of superglue.