Looking at the photos I've provided would you agree that it will be pretty straight forward?
Isolate power, remove front cover and back plate from the ST9400C, transfer the wires from the back plate to the corresponding terminals on the Hive back plate
N..L..3..4
Remove thermostat and backplate from the wall (there are only 2 wires connected to the thermostat) join the 2 thermostat wires together with a suitable connector and tuck away in the recess then mount the Hive thermostat over the top.
Thanks in advance for letting me know if I've missed anything or if any photos are required.
Isolate power, remove front cover and back plate from the ST9400C, transfer the wires from the back plate to the corresponding terminals on the Hive back plate
N..L..3..4
Remove thermostat and backplate from the wall (there are only 2 wires connected to the thermostat) join the 2 thermostat wires together with a suitable connector and tuck away in the recess then mount the Hive thermostat over the top.
Thanks in advance for letting me know if I've missed anything or if any photos are required.