UPDATE 
I've been around the block a few times in IT, in far too many years.
Bear in mind that the system worked, the DM connected, and we had no issues, before the carefully planned power down/power on. It was post-power on that issues occurred.
The MP40 has a 10/100 network interface. It had been connected to a Juniper EX2300 10/100/1000 POE switch.
Out of curiosity I pulled an old HP 10/100 switch out of the 'just-in-case' box, hooked it up, connected the MP40 (OS 7400) to it and.. It works!
Why the connection should have been lost, I do not know. I'll have a look at configuring a port on the Juniper to remove POE and Gigabit functionality, and see if connectivity is retained, but for now - it'll be the single connection in a 24-port switch.
I've been 'round and checked all IP phones - all are working as expected.
While the very useful suggestions I've received here weren't the answer, it has (with hindsight!) been a great experience, and I thank you.
