OK thanks, good job I checked. In that case
To remove the old thermostat find the origin of the black and grey wires in this cable that you have highlighted...
...where they terminate at the wiring centre on the left in your photo. Note where they go, and then remove the old thermostat cable and the old thermostat receiver completely. Then join together the two terminals where you have just removed the black and grey wires from.
The brown and blue L & N wires are just disconnected.
To remove the old thermostat find the origin of the black and grey wires in this cable that you have highlighted...
...where they terminate at the wiring centre on the left in your photo. Note where they go, and then remove the old thermostat cable and the old thermostat receiver completely. Then join together the two terminals where you have just removed the black and grey wires from.
The brown and blue L & N wires are just disconnected.