Comment 12 for bug 1377624

@ Pitti thanks for the hint:
I've added -x to the udev postinst

$ sudo dpkg --configure udev
Setting up udev (204-5ubuntu20.9) ...
+ set -e
+ update_hwdb
+ udevadm hwdb --update
+ invoke-rc.d udev restart
 * udev requires hotplug support, not started
   ...fail!
invoke-rc.d: initscript udev, action "restart" failed.
dpkg: error processing package udev (--configure):
 subprocess installed post-installation script returned error exit status 1
Errors were encountered while processing:
 udev

And clearly /etc/init/udev.conf was not on my system.

I'm suspecting OVH.co.uk provider miss-configuration.

$ sudo apt-get -o DPkg::options::=--force-confmiss --reinstall install udev:amd64

Does not work - I get:
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following package was automatically installed and is no longer required:
  python-twisted-names
Use 'apt-get autoremove' to remove it.
0 upgraded, 0 newly installed, 1 reinstalled, 0 to remove and 0 not upgraded.
4 not fully installed or removed.
After this operation, 0 B of additional disk space will be used.
E: Internal Error, No file name for udev:amd64

(will file a bug about that)

So finally I did:

""""""""
WORKAROUND

$ apt-get download udev
$ sudo dpkg -i --force-confmiss udev_204-5ubuntu20.9_amd64.deb

""""""""

And that has resolved things:

$ apt-get download udev
Get:1 http://ubuntu.mirrors.ovh.net/ftp.ubuntu.com/ubuntu/ trusty-updates/main udev amd64 204-5ubuntu20.9 [734 kB]
Fetched 734 kB in 0s (14.5 MB/s)

$ sudo dpkg -i --force-confmiss udev_204-5ubuntu20.9_amd64.deb
(Reading database ... 26276 files and directories currently installed.)
Preparing to unpack udev_204-5ubuntu20.9_amd64.deb ...
Leaving 'diversion of /bin/udevadm to /bin/udevadm.upgrade by fake-udev'
Unpacking udev (204-5ubuntu20.9) over (204-5ubuntu20.9) ...
Setting up udev (204-5ubuntu20.9) ...

Configuration file `/etc/init/udev.conf', does not exist on system.
Installing new config file as you requested.
udev start/running, process 13434
Removing 'diversion of /bin/udevadm to /bin/udevadm.upgrade by fake-udev'
update-initramfs: deferring update (trigger activated)
Processing triggers for man-db (2.6.7.1-1ubuntu1) ...
Processing triggers for initramfs-tools (0.103ubuntu4.2) ...