How do I make somethign look steeper whilst looking down it? Seems whenever I set the helmet/bar/bike/fork/chest strap camera up the video imediately makes eveything appear completely flat, even if in fact its buttock clenchingly steep.

not easy to do

very wide lens is the only way really. As soon as you point a camera level with the ground then the gradient effectively appears to be nowt.

you need to shoot side on or cut in set up shots showing the trail set in it's terrain

hmmmm, sounds familiar, closest I found was to put it low down so you could hear the freehub clicking as the bike speeds up out of corners and the tyres buzzing into the bottom of berms or g-outs.