An update here.
Thanks to the help of a couple of users here I've managed to get it fixed.
The software on the GPS needs to be updated to version 3.60, which has recently been released. This corrects the problem (or at least, it did for me).
Connect your GPS to the serial port. Go here and download the "WebUpdater" software :
https://support.garmin.com/en-GB/?productID=8705&tab=softwareThen it should just be a case of starting up the software and with the GPS connected it should tell you there is an update and offer to install it.
That fixed it for me.