The old stat has to go.
In fact, I assumed that it already had gone, and that the Brown, Black and Grey wires shown connected to the Hive backplate were those that used to be inserted in the old thermostat terminals 1, 2 & 3. They should be, regardless of which method of installation is used.
In fact, I assumed that it already had gone, and that the Brown, Black and Grey wires shown connected to the Hive backplate were those that used to be inserted in the old thermostat terminals 1, 2 & 3. They should be, regardless of which method of installation is used.