When I bought ML originally I did save something. It's a folder called : Install OS X Mountain Lion.app with a size of 4.07GB, so it might be what's needed. Question though is : What to do with it ?
Ah hah! You are in luck then. Save that file! That should be a 10.8.0 version of ML. I would use this free app along with that file to make yourself a USB key installer. Then option key boot from the USB key and format with Disk Util like I described earlier.... and install away. That will give you a clean install of 10.8.0.
If you don't have a USB key, you can make a 6GB partition on your external USB drive and use the same app I linked to put the installer there. Then option key boot to that partition on the external.