In the end I went with an Eduan Float Valve in the top tank and using the pressure sensor on the pump to stop when the top tank is full and it works very well.
I initially tested it with a cheap ball valve from the local hardware shop, but the water throughput was appalling. The Eduan valve is a huge improvement.
I still need to do a fixed power installation and add my float control to the bottom tank to only start the pump if there is water to pump. At the moment it is still manual with a lead cable.
And the veggies love the rain water
