Having not touched my system for several weeks, I caught up on the latest updates. When I restarted, the ZHA integration failed to initialise the SONOFF dongle and the Zigbee network is not working. Bit of a disaster!
I could understand the error message (below) but by tracking back through my backups I showed that Home Assistant core 2023.2.3 was the culprit. This is a very disconcerting as until now I had installed all updates without hesitation. It seems I need to be more circumspect 
Logger: homeassistant.config_entries
Source: components/zha/core/gateway.py:174
First occurred: 20:08:15 (1 occurrences)
Last logged: 20:08:15
Error setting up entry Sonoff Zigbee 3.0 USB Dongle for zha
Traceback (most recent call last):
File "/usr/src/homeassistant/homeassistant/config_entries.py", line 382, in async_setup
result = await component.async_setup_entry(hass, self)
File "/usr/src/homeassistant/homeassistant/components/zha/__init__.py", line 111, in async_setup_entry
await zha_gateway.async_initialize()
File "/usr/src/homeassistant/homeassistant/components/zha/core/gateway.py", line 189, in async_initialize
raise exc
File "/usr/src/homeassistant/homeassistant/components/zha/core/gateway.py", line 174, in async_initialize
self.application_controller = await app_controller_cls.new(
File "/usr/local/lib/python3.10/site-packages/zigpy/application.py", line 163, in new
await app._load_db()
File "/usr/local/lib/python3.10/site-packages/zigpy/application.py", line 80, in _load_db
await self._dblistener.load()
File "/usr/local/lib/python3.10/site-packages/zigpy/appdb.py", line 581, in load
await self._load_unsupported_attributes()
File "/usr/local/lib/python3.10/site-packages/zigpy/appdb.py", line 634, in _load_unsupported_attributes
ep = dev.endpoints[endpoint_id]
KeyError: 2
(I tried pasting the whole log, but it is apparently too big?)
Reverting to core 2023.1.7 fixed it