just failed a 3000 word essay on marketting and branding. I don't think I should have failed as I feel I covered most areas and submitted a good piece of work.

can I ask for this to be double checked

Yes, but when you do, be ready to tell your instructor WHY you believe it deserves re-grading. Point out all your strong points.

The instructor may or may not agree, but you have to go in with more than simply "I covered most areas."

Wait! Did you write "most areas"? (Not all areas??) Hmmm. Well, it's worth a try if you point out your strong points.