I didn't want to miss the game so I left it till this morning to fix.
After connecting the laptop to my wired network this morning, I immediately got a clue about what had happened - even before I had made the necessary changes in the interfaces file to bring up the wired interface, I noticed that the laptop was asking for, and obtaining a DHCP lease, from the DHCP server that I run on my workstation.
In short, there was an application, in this case NetworkManager, that had come in with the last upgrade and taken over the function of networking. It was doing things that were not specified in the interfaces file. This was something that had happened without my intervention.
It took a few minutes to remove NetworkManager and its associated dependencies and immediately sanity was restored. Everything worked as it had before this brain-dead app descended on my laptop.
On the NetworkManager home page it says: "Networking on Linux can be painful, especially in comparison to other operating systems. You should never need to use the command line or configuration files to manage your network (unless you want to!); everything should "Just Work" as automatically as possible and never stop you from doing what you want to do.
And there's more marketing spiel: "NetworkManager attempts to make networking as invisible as you want it to be. Whether at home, work, or on the move, NetworkManager automatically connects to the last network you told it to connect to. From wired to wireless to mobile broadband to Bluetooth, NetworkManager has you covered."
Take it from someone who's been burned: this is high-grade bullshit. Whoever runs this project should just change that to the exact opposite. NetworkManager is exactly the kind of app that will drive people away from GNU/Linux - it makes things look very much like the voodoo world of Windows.