Once again, after an update, the NM applet refused to co-operate and started showing all connections as ‘unmanaged’.
This relates to Bug #280417, and the fix was rather simple:

Whatever updates occurred in the last day set all the devices to managed=false in /etc/NetworkManager/nm-system-settings.conf

Change that and run sudo killall nm-system-settings and it’ll start working again.

I’m posting it here more as a note to self, than anything else.

And as I am in ‘note-to-self’ mode, it may be worth pointing out that, to configure a static IP address one has to edit:

/etc/network/interfaces

auto lo
iface lo inet loopback

auto eth0
iface eth0 inet static
address 192.168.1.50
netmask 255.255.255.0
gateway 192.168.1.1
broadcast 192.168.1.255
network 192.168.1.0

/etc/resolve.conf

nameserver 192.168.1.1
# The following are the DNS servers for VirginMedia:
nameserver 194.18.4.100
nameserver 194.18.8.100