i just got to the bit fussel was originally talking about. you can get across the first log ramp with the bike, but then there is one that is "down" and you cant cross it.
eventually, i figured it out. here's a vid [Warning: vid is spoiler!]
BTW, i never did throw the switch up at the top, the one with more missing heads. did i miss a head, or is it one of these newfangled; to be done on the return pass tricks this game is throwing (like the krakan/octopus high up thing?)
opps, my posts gone mad. i accidently posted some corrupt messages. can someone delete them, i cant find a delete? thanks.