Almost done. I've made up a working prototype. I'm happy with the mechanism though the layout needs a little more work. 

This version only has a ratchet on one side but I can see that it'll work once complete. I've changed the ratchet housing again. Back to double thickness card but this time wider to that the ratchet itself fits fully inside the housing. It seems to be more stable like that. I've reduced the diameter or the gear a little and reduced the number of teeth to seven.

To complete the model I'll add a ratchet to the other side and a second linkage. That's for tomorrow though, I'm off to experiment with html 5 animation. Fun!