Honeywell ST699 with Drayton digistat SCR

Above is my present heating system, connected to an outside oil fired boiler. I've been trying to install the tado Smart Thermostat V3. i have come unstuck on the extension kit. I cannot work out what I am supposed to do, and the 'technical support,' is not helpful. Has anyone got experience of this system? And if not, does anyone know how I get my money back?