I'm getting so frustrated! I have killed all the bad guys, climbed the ladder where there is a big box at the top, climbed the wall, jumped over the gap where there is meant to be a zip wire down to the crane, it's the only way across but I have no zip wire! It was there when we first came to that but but my son fell off in the next section so it started us back near the ladder again and now no zip wire! I've reloaded from last check point, tried to go back as far as I can to see if that helps but nothing! Please tell me I don't need to start back at the beginning of the game!!! 

Hi, Liz. That sounds like a nasty bug. You have my sympathy. If you're playing the PC version, you can download a save file from my site. Files and instructions are at  have http://tombraiders.net/stella/savegame/TR10saves.html. If you're playing on the Xbox, you should be able to reload a backup save by following the instructions here: http://forums.eu.square-enix.com/showthread.php?t=182989&p=2182925#post2182925

Hope that helps. Good luck!