iI want to use Slate chippings and ornimental slabs etc. How much sand do I put down to put the slabs on. ( how deep ) Also do I need to put a membrane down to stop weeds

Most patios need a gravel base under the sand. You only need about an inch of sand on top of your base. I use 1" copper pipes and a board to level. That way it is uniform. You don't need a barrier if you fill between the stone with cement or polymeric sand. If you just use sand, I would use a barrier, but most stuff will grow on the sand above it anyway. you could also use ag-lime instead of sand- it is less likely to "grow" things in it....good luck!