Update to the wiring in the control box, in the old system there were two relays in line with the heater supply, one was controlled by the thermostat and one by the safety circuits, one was driven by low voltage and the other by mains. It seems logical to just have one circuit and then I could just use a light duty relay to enable the circuit to enable the heater.
After I had wired this it dawned on me why they may have done it that way.
It's an additional safety feature, in case one of the relays contacts stick or weld together, so back to the two in line relay design.