Tjclark,
Another check is to use your working Arduino as ISP and upload the Blink example. If it works you have probably fried the 8u2 or 16u2? USB chip that your board uses. Since Pin 13 LED flashes the 328 chip might still be working.
BTDT, now I have a board that won't work on USB. Tried changing the 16u2 out on my Uno, no luck with smt soldering.
TomJ