no unfortunately i did not backup my HD - however im not that fussed as It was only a spare machine for surfing the net and emails so thats ok
Can I do a roll back without the backup?
If you have a bootable usb drive with Mountain Lion you can do a reinstall, here's how:
Create a bootable install drive for OS X Mountain Lion, if you don't have one already.
With the boot installer drive connected to the Mac, reboot and hold down the Option key
Choose the Mac OS X Installer startup volume from the boot menu
Select Disk Utility and choose the hard drive you wish to format, click the Erase tab, and then pull down the Format menu and select Mac OS Extended (Journaled) as the type, name the drive if you choose to do so
Click the Erase button and let the drive format this is the point of no return
When finished, quit out of Disk Utility and now select the Install Mac OS X option from the menu
Choose your freshly formatted hard drive and install Mountain Lion
When the Mac reboots you will have a clean installation of Mac OS X 10.8 to work with.
Now this will be a fresh copy of ML, so you won't have any of your apps, data, etc.. not like a time machine rollback. So at this point you can either import files and apps from a backup, or just start anew.