Right well I installed Cyanogen 11 this morning using the Installer, I did have an initial issue with it not connecting to the phone and telling me to try different cables, disable AV etc but non of it worked. Found a fix later on that I've posted below.
Other than that I'm really impressed so far, the phone is much faster that the stock Samsung one (and I had reflashed it only a month ago). Chuffed
EDIT: I'll also add that everything but GPS worked straight away, Google Play was there and no app crashes so far. Handy that you can turn root on and off as well I like that.
Waiting for update on GPS issue but other than that I'm very impressed with CM11.
FIX for not connecting issue
1. Put your phone manually in Download Mode (pwr + vol down + home)
2. Plug device in to PC in Download Mode
3. Download and extract the 7z file here Download libwdi from SourceForge.net
4. Launch zadig.exe. Select Options > List All Devices. Select "Gadget Serial" from list and click the "Replace Driver" button. This will force in a driver for the device when in Download Mode
5. Close zadig.exe and reboot your device. Retry the installer.
Sprint Galaxy S3 "cant Talk To Your Phone" Message - CyanogenMod Installer - CyanogenMod Forum