Now stuff that I tell you on this blog is what I am learning as I go. This is no exception. I had no idea what a Decoupling Capacitor was. I kind of knew what a capacitor was. Its similar to the hot water tank in the loft in its function. As you use hot water, it refills the hot water boiler, then it in turn is refilled from the mains water supply. Similarly if you connect a capacitor between the +5V and GND pins on the Z80 CPU it will fill with charge so that when the CPU starts to drain the battery by turning pins on and off the capacitor can take up the slack and keep the CPU with voltage. I used a 0.1uF (microfarad) capacitor. It needs to be as close to the CPU as possible so I just straddled the capacitor directly over the CPU.
Ideally you'll have one on every logic chip, but I opted to just put one on the whole circuit so just decoupled the main +5V supply and ground.