Thanks for the advice guys. I just assumed that the instructors knew what they were talking about (not a crazy assumption to make!) so I didn't look into it again until recently, but I will definately investigate more.
I guess one of the things that made me not jump on the day was that we'd done our whole course with one guy, who was very experienced and reassuring and knew us all, but when it came to the jump he decided to be the guy on the ground, not in the plane, so the guy on the plane we'd met v briefly and he wasn't nearly as experienced (about 250 jumps compared with 4000 in the first guy), and said he hadn't been up with students that many times, which is obviously fine if you want to jump but not so reassuring if you're not sure! And when I didn't jump he had to ask the pilot what to do next because he didn't know... which didn't fill me with confidence! So there I was, looking over the door of the plane, and this virtual stranger was saying 'you'll be fine' and I was thinking 'who the hell are you?!'
Anyway, I was very scared of heights at that time so maybe skydiving wasn't the best sport to try out, but since then I've been taking lessons as a pilot so that has made me a lot calmer about them- and also made me have to accept that one day I might have to jump out a plane if the piloting goes a bit wrong!