This was a bit more long winded than I thought it would be, but thanks to a pretty reasonable guide here, I managed it without many issues.
The problem was simple in that sockets, quite rightly, have a different device group id that lighting, so while I was able to switch the socket off and on state was no propagated properly via MQTT. The fix was a simple one to duplicate a small amount of code in TradfriDevice.java and change 3311 (lighting) to 3312 (sockets).
The big problem was working out what gradle was and how to use it .. but once I had found ./gradlew –info I then found ./gradlew tasks and that lead me to ./gradlew buildNeeded which did the trick.
Here is my pull request.