I'm not sure what's causing this. In the latest Chrome update, Google have started automatically disabling extensions that don't come from the Chrome Store. However, installation by dragging and dropping the script into the chrome://extensions window still works fine for me, and the extension is still there after quitting and restarting.
Potential workarounds could be to enable developer mode in chrome://extensions, or to install Tampermonkey from the Chrome store and to use that to install GME. Let me know if you have any luck!
Thanks for the reply.
I've tried enabling Developer mode (as I'd read somewhere that this stopped the disabling) but it is still disabling them. I currently install GME by dragging onto the Extentions page; I'll try the Tapermonkey later and see if it makes any difference.
Loading via Tapermonkey has cured the problem -