Thanks WCDev,
I had already downloaded the FTDI driver but didn't get anywhere with it. I may not have installed it correctly (or completely) or it may have been the wrong one. However your suggestion to look up the System Information got me on the right track. The dongle is a Unitek Y-105 but there was no Mac driver on the Unitek site (even though it said that it worked with Mac). However the System Info showed the device as a "Prolific Technology, Inc. 2303D".
I then found this link that explains almost everything -
http://plugable.com/2011/07/12/installing-a-usb-serial-adapter-on-mac-os-x. The screens in this link match exactly with what I saw.
Of course it is a generic instruction so when you get to the end you need to start WeatherCat and select the port named "/dev/cu.usbserial" .
Everything is now working brilliantly.
Thanks for pointing me in the right direction. Hopefully this will help someone else in the future.