I’ve answered my own question. I asked why my Sonoff device was not being controlled correctly by a Home Assistant MQTT switch using multi-level identification, for example:
home/inside/switch/kitchen/worktop_lights
I found this thread on Google Groups which suggests the Sonoff-Tasmota firmware does not allow for multi-level identification of MQTT clients and instead requires each Sonoff device to be identified by a unique name (e.g. cmnd/kitchen_worktop_lights/POWER).
Chris