Dopez is so right!
My experience, I basically did a narrative form of video so I guess it's a little different from yours. I did around one and a half hour total of filming, all editted down to about 7 and 1/2 minutes. Basically, when I was filming mine, I did a lot of repeated takes to get the ideal shots.
I see where you are going writing an essay and a script. Just to make sure, are you doing this because you want to do a voice-over, something like they do on TV when they have a narrator? I think it would be great if you do both the essay and the script. Or write an essay first and edit it into a script. but like Dopey said, always finalised it as a script.
Some other points to consider is that
1) What sorts of video camera are you using, a digital or an analogue one? If you are using an analogue one, make sure you have the right cable that allows you to convert the footage back to digital in order to edit on computer.
2) Make sure you have a good editing software. In this case, you are looking at either Adobe Premiere or Final Cut Pro. I have used both for mine because of different problems. I think that Final Cut Pro is easy to use for editing the video part of the footage but because I was working on an imac, the sound track didn't go very well. So I actually tranfered my timeline to Adobe Premiere. Adobe is also great but i didn't use it initially because the transitions are harder to do than Final Cut Pro.
3) Since you are doing a documentary, then I guess you would have to pay extra attention at the sound. that's because they have said in the 2002 marking guidelines that, some candidates have poor sound. This detracted the overall product. Use a good mic or maybe, even try a synchronised sound like they do on TV (that means that you ask your actors to go to a voice room to re-do all the sound accordingly). Make sure that the sound for different parts of your video is even.
i think they have the syllabus on the board of studies site.
also check this site out for some filming handy hints (although it mainly targets at people making short films)
www.exposure.co.uk
Just to let you know that, we are always here to help