Hey KT,
Most of what you're taking looks fine. When you say flapjack, do you mean pancakes (sorry, Canadian)?
I recommend you take nutrient dense food with you. Personally, I would advise against muffins since they take up a lot of space, generally aren't very nourishing, and they won't travel well.
I would pack the same as if I were going on a hike for as many days as your festival. I'm assuming you'll have access to clean water. If not, I would get a good water filter.
If you're going to freeze food to take, eat that the first night.
Here's what I would take for food...
Equipment:1. If you're trying to save money, it's easy to build a rocket stove. (
http://www.instructables.com/id/How-to-make-a-Rocket-Stove-from-a-10-Can-and-4-So/) Very easy to make and very fun to build and use. You can assemble it on site. Make sure you test it before you leave. This is so low key and only burns twigs so chances are no one would notice.
2. A water container or two minimum 1 liter, preferably 1.5 liters
3. A small pot with no handle (no handle saves space and the pot doubles as a dish)
4. A spoon
5. A multi-tool or pair of pliers (to move the hot pot and they have 1000 other uses while camping)
Food:1. Trail mix (only dark chocolate in it if any and not very much)
2. Hot chocolate powder
3. raisins and dried cranberries (high energy)
4. dried fruit (apples, bananas, etc)
5. Tuna (very high in protien)
6. Olive oil (keeps you feeling full)
7. Almond milk (very sealable package)
8. Tea (generally peppermint and licorice) (this combination is extremely calming if you need sleep)
9. Sausage (pepperoni or salami doesn't need refrigeration)
10. If you must have bread take bagels (they pack better and are more dense)
11. Instant oatmeal
12. Soap! (often forgotten)
I hope this helps a bit.