I am thinking of replacing the concrete base outside my back door. It is uneven ‘ cracked / missing, basically shot but covered by decking currently. There is a step down out of the back door onto the ground and then back up onto the existing concrete, then up again onto the decking, circa 100mm per step up or down. Now I have changed the door I could bring the decking right the way to the back door and make it look 10 times better and get rid of the steps. Apart from light foot traffic the decking would have an inflatable spa on it though through the summer months so needs to be solid. What would I need in the way of a base, area is circa 4.2m x 2.5m. I am guessing I might as well just put down a replacement slab, nice and flat ready to put new decking on top of. Would it need to be 50mm slab with 50mm hardcore underneath or could I get away with just a slab. Or maybe just some sort of MOT base to a certain depth?