Someone suggested using a ratchet, so I decided to experiment.
You can see the results so far, and although it's a bit rough it seems to do what I intended
Some of the movements need re working, and the cams which opperate the coffin lid and zombie need changing in size so the lid stays open longer but so far so good.
I made the spiral shaped cam by drawing 2 circles, adding lines every 20 degrees and then marking a point x cm from the centre, each subsequent point slightly further away and then joining them. Not mathematically perfect but works.