Problem with serial driver for CH340G board
Using different search criteria I found http://kiguino.moos.io/2014/12/31/how-to-use-arduino-nano-mini-pro-with-CH340G-on-mac-osx-yosemite.html

This indicated a newer driver for OSX 10.9 and above at:-
http://kiguino.moos.io/downloads/CH34x_Install.zip

It is now working OK.

PS This was probably a Mac question, but I will answer as it is mainly of interest to Arduino users.

PPS The board worked on my Raspberry Pi which had the driver installed.

WARNING

The driver worked on OS X 10.11 but after upgrading to OS X 10.11.2 when I plug the USB cable into the Mac it causes a crash. This may or may not be related to the upgrade but after this happened twice I gave up.