In this post I will explain a bit more about the Mk 1 project.
The picture below is the main board with the Picaxe 08m2 mcu chip. It also has a 12V-5V 78L05 regulator and a P36NF06L MOSFET to control the pump.
Here is the soil moisture sensor. Two lengths of galvanised steel wire. Sophisticated!
And here is the pump. When installed, it was connected to the end of a length of hose and sunk into a water butt that is fed by rainwater from the greenhouse roof.
Power is from a 12V SLA battery, charged by a small-ish 12V solar pannel. No charge regulator was used, because I had read that SLA batteries are reasonably resistant to overcharging, especially with small pannels.