James, I'm not sure how many flies might have larvae that closely resemble BSF. Blow fly maggots are smaller and the pupae are red, and house fly maggots are much smaller than BSF. Also the fact that you're finding these larvae in fish pellets makes it likely that you've got BSF. In south Georgia people often call BSF larvae "meal maggots" because they're commonly found in exposed feed that has gotten moist.
The best way to identify the species is to observe your feeder on a hot sunny day, after noon. If you have active BSF larvae in the feeder you can be pretty sure that there will be adults attracted to it as well. The larvae are a powerful attractant to adult BSF and that way they maintain their dominance at the food source.
It's really easy to identify an adult BSF. They are 3/4 to an inch long and have white legs. You can stand within a few feet of your feeder and they will still land on it. If anyone asks why you're standing there staring at a fish feeder, just tell them you're doing scientific research.
In the U.K. maggots are a very popular bait and most bait stores stock them. Here's a web page that describes rigging with them: PleasureFishing.com
