Arduino Leonardo on Win 8.1 COM port disappeared
Strange, it's resolved itself/I resolved it. I don't know whether what I did was actually responsible for solving this but I'm back in business:
* I reset the board so that the bootloader appeared as a com port
* I set the arduino IDE to use the port that appeared for the bootloader
* I uploaded a simple sketch and pressed reset during the "upload phase" so that the port appeared during the upload
* the upload completed successfully and the arduino was back in the com port list

I'd still be interested to understand what I did that caused the problem in the first place but I suppose something was corrupted on the board?