Bypass Thermostat Heating UK
Figure 1. Most likely original circuit.

If you were really fortunate the live feed to the thermostat would have originated at the original switch. You could then have connected it directly to your new controller. Judging by the photos there is only one live wire so this isn't the case.







Figure 2. New live feed.

Options:


Get a new live feed from the original breaker to the controller.
Break through the wall and restore the original connection. 
Try to find the wires above ceiling by lifting floor in the room above or in the attic. Isolate the power, cut the wires and join them together again.