CANCEL - Corrupted plugin, can't reinstall

Please delete this one, having scanned through the code for the message, I've realised I forgot to rename the folder back from my change for the version no. The Activate didn't like it...

Marketpress had been running fine, but suddenly it was refusing to store the price info. I uninstalled it as a first option, but when I went to reinstall it I got this error:
"There was an issue determining where MarketPress is installed. Please reinstall"

I've tried any number of things without success:
1) Delete and reinstall (error above)
2) Latest Marketpress version install (same error)
3) Rename everything in the plugin from Marketpress to 'Marketpress2' (same error)
4) Deleting every single plugin (in a test copy of the site) (no change)
5) Checking the active_plugins field on wp_options (is 0)
6) Checking if it happens with other plugins (No)

I'm assuming there are no strange installation requirements - I don't remember any. I really don't want to rebuild the site from scratch...

Any ideas?