Alastair, Are you trying to get the 64-bit or 32-bit? Because X17-24208 is 32-bit but X17-58997 is 64-bit.
Once you get the correct bit size it is important to verify you got the entire ISO. The 64-bit file should be 3.24MB. The USB tool has to be installed and then when you run it, try running as an Administrator, see if that helps.
If you are sure you have the entire ISO and the tool doesn't work even as an administrator then you will have to create your own bootable USB stick and then simply expand the ISO on to it without the USB Tool. Go back to the post
HERE for instructions on making a Bootable USB without the tool.