If you're making bad things up about your fictional company, I think they'd have to prove
first that everyone assumes your fictional company was them and
then believed the bad things were them too. And if what you say is true, then you haven't libelled them and they can't object (which wouldn't mean they wouldn't try, of course!). Also, a negative opinion isn't libel: you have to damage someone's reputation to libel them - i.e., saying So-and-So's dresses are ugly is fine, saying they're badly made might not be libel, saying they're made of nylon but labelled as silk would be.
The Society of Authors could advise, and even if you're not a member they have a guide to libel for a couple of pounds:
http://www.societyofauthors.net/soa/section_page.php4?hp_nav_id=16&urlsection=Publications
Emma
<Added>I see from your last post it's non-fictional, but it still holds that opinions that clearly are opinions aren't libel, nor is the truth. Journalists deal with these issues all the time - the NUJ might have advice too.