It depends on a few things. Grades obviously matter the most... so you need to be close.
However, if there are spaces left on the course after everybody who has met their offers has been accepted, they ususally give priority to someone who had an offer but did not meet it and say got.. 1 A 2 B's.. than someone who rings up randomley through clearing with 3 A's...
Or at least thats what manchester told us.
So I'd say you have a good chance if you get OK grades and your course isn't full.