Sub Base with Core Gravel grids

If your Base is of sound quality and no big pot holes in would put down weed proof membrane then over the top a dust or sharp sand level bed for the Grids to sit on. The Grids should come with pins to hold Grids in place. There are different thickness Grids that will take traffic should state when looking. To save on cost of decorative stone you could half fill the core Grids with 10mm clean stone first then finish off with your 20mm decorative stone.hope this helps.

A small amount would be wise to get the area level as the grids aren't strong enough to take the weight of vehicles and will deform to the shape of what is under them. You could use sand or ballast, but either way it gives you a nice flat base to start from, just get it level with a board across the space and wacker plate it down and fill in any bits that go low until it is all level.