You can only restore a database into the same version it was saved from.

When you do, it will contain any database corruptions that were in the other site at the time of the restore.

So, if your database backup was taken before your corrupted the old site, you can make a new install of the same c5 version and import that database to it. You will also need to copy across files, packages and root level overrides.

( For the future, have a look at Backup Voodoo http://www.concrete5.org/marketplace/addons/backup-voodoo/... )

Then, when you have the clone installed at the old version, you can begin upgrading (but take another backup first). There are many other precautions to take when upgrading a site, so read back through the howtos and forums to get an overview of good practice.

thanks - how do I check the exact version used when I backed up files given I can't now get in there?

If you get the .sql backup file into a text editor, you will be able to read the site settings from the Config table backup text.