Paul,
You have followed the following Windows 8 advice
.From the Metro Start Screen, open Settings (move your mouse to the bottom-right-corner of the screen and wait for the pop-out bar to appear, then click the Gear icon).
2.Click ‘Change PC Settings’.
3.Click ‘General’.
4.Scroll down, and click ‘Restart now’ under ‘Advanced startup’.
5.Click ‘Troubleshoot’.
6.Click ‘Advanced Options’
7.Click ‘Startup Settings’
8.Click Restart.
9.When Win8 restarts type the number next to "Disable driver signature enforcement"
If still not working are you connected to the computer via a USB hub or direct to the computer. If via a hub try direct to the computer. You may also require to have a look at your device manager for the USB ports to see if you require to change something. If still nothing have a look at your USB cable it may be knackered.
Alan