- From stock, booted into recovery, then replaced TWRP with CWM. Rebooted into CWM.
- Tested sideload from CWM. Still non-functional. Made sure cmd.exe was running with administator privileges, still no difference. Time for _buntu!
- Installed Xubuntu. Connected to wifi, downloaded
- Sideloaded and did a dirty flash of CM11 (didn't want to wipe without first confirming I could flash and everything was functional). Issues (as expected) such as slow loading but able to boot up.
- Decide it's time to push my luck and flash TWRP - this goes without a hitch.
- Test sideload. Still working, so perform full wipe and factory reset. Then sideload CM11 + GApps.
- Reboot into CM11. Still some issues so flashed the CM11 kernel using fastboot (not sure if TWRP does this automatically, but since the boot up improved a bit, I'd guess not).
- Now have a fully functional phone, albeit some of the changes from CM10.x to CM11 have disorientated me a bit...
Thanks for your help man. As with a lot of things this turned out to be a dopey Windoze issue than a problem with the phone or adb etc!