There's been a lot in the "news" lately about Purina Beneful dry food having maggots, here's The Consumerist
Are 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.