Sorted it all out. You just need to delete all the tables starting with “bb” and re-install as usual.
Thanks, everyone.
The site’s run by an aquaintance of mine – I get free hosting from him. I’ll take a looksie as to what value I put for the user prefix.
Watch this space.
I’m pretty sure it was wp_user, although it may have been wp-user – I can’t remember exactly.