In reply to Oli:
I bought some 25mm webbing today and did a proper slackline. At least I think it's a proper slackline, it's just I don't think it's as hard as people make out. Either I have an innate talent for these things or I've found a way of setting them up to suit beginners. I do suspect it's the latter.
The line is probably only about 15-20 feet long. It's about 3 feet high at each end and sags down to within about 6 inches of the ground when you walk on it.
It's seems pretty easy to stand in one place, walk along it, turn around and squat down low on it.
I'd like to sit on it but I feel the need to drop one knee really low and the line isn't high enough. I think I'll make it a bit higher next time.
It can't be as hard as tight-rope walking, because even if you balls up and manage to make it so that your centre of gravity isn't between your feet, the line will soon wiggle to a position where all is good again.