First, I wouldn't compare Valiant with Wallace and Gromit. Wallace and Gromit are fairly well-known characters and with the Halloween theme of the movie, it should hopefully play decently.
Secondly, what is your nearest competition? Did they play 40 Y/O Virgin? Whenever you play a movie that's been out for awhile, you always run of the risk of losing business because people who want to see the movie right away will go to the competition and see it right away.
And thirdly, what type of movies tend to do well for your theater? I know for our single screen, we tend to do well with children/family movies and we've done very well when we've opened movies like The Incredibles, Shark Tale, and Shrek 1 & 2 and only had to play them for 2 weekends. Hopefully this helps.