Hi PeterJMelb,
re-imaging in the IOS world is called a "restore", but it is quite different from the "restore from backup" option in iTunes. Very confusing to call them both a restore, but that's the way Apple did it!
A "restore" will install the latest version of IOS on your device and set it back to factory defaults. Once that process is complete you can choose to either continue as a "new" device or restore your previous backup to get your apps, settings, media, etc. back on there.
To restore, connect your device to you PC, open iTunes, click on the iPad device in the left hand panel and choose "Restore". There may also be an "Update" option available on the same screen. This upgrades your current IOS to the latest one if you are not up to date. It is a genuine upgrade and does not wipe out anything on the device, as opposed to a restore, which is akin to formatting the disk on your PC and reinstalling the OS.
Apple have security controls in place to ensure you can only install the latest and greatest IOS for your device (in your case 5.1.1). You can't normally choose to install a previous one (or downgrade the device), however there are some hacks which enable this but only if you have prepared in advance by saving away the digital signatures for old version of IOS when Apple was providing them. In your scenario, you won't have any of those saved away.
So, to recap, connect your iPad to iTunes, and press "Restore" to set the device back to factory freshness on 5.1.1
If you run into any problems, we have a failsafe tutorial here, which resolves all common errors with a restore -
http://www.ipadforums.net/jailbreak...lsafe-method-restore-current-version-ios.html
Hope that helps!