There's been a lot in the "news" lately about Purina Beneful dry food having maggots, here's
The Consumerist's article:
http://consumerist.com/5052707/purina-isnt-bothered-by-dog-food-infested-with-fly-larvae-maggotsAre you saying you found maggots in the Pedigree food you have? If so, can you please post the specific product name; the lot code; and the best by date so others can be alerted?
Have you notified the store you purchased it at? Make sure you speak to a manager and file a written complaint. Give them the same information I listed above so they can inspect those bags.
It sounds like you have already contacted Purina, and it doesn't surprise me they are in denial about any problems.
ETA: If you can upload a photo of the food showing the maggots that would be great. The only way to get the word around is through forums like this.