Hi, I don't know what I've done but I've reinstalled this plugin many many times and restarted the server many many times... It's just stopped creating the kits.json file and as a result I can't use any commands in game... It's as though it's not installed... 

Any thoughts?

The kits.json is stored in the data folder. I would suggest deleting everything associated with kits, from all folders, then re-installing and starting from square one if you can't get it working.