I would ask the artificial turf suppliers what they recommend?

I suppose that ythe plywood would make a cheap, sacrificial base for the artificial turf. The plywood would rot rather than the decking.

I would be inclined to use ashfelt,this will protect the dcking and the better the ashfelt the longer it will last and the better job it will do. if there are gaps in the decking then of corse a thin ply wood first, it dose not have to be thick it is more to stop the bords cuting the ashfelt.

That sounds a better Idea Cliffo.