Hello....I am planning on laying limestone slabs....I guess I need to use gully top as level guide?....planning on drilling holes in gully for aco drains If ok to do it?...1 aco coming out right as per photo across garden....the gully top is 100mm below damp course....im going to need to have a gradient towards back of house ...4meteres so will loose another 40mm approx...I will put in French drain maybe 3/4 inches....but the top of slab at back will only be about 60mm below dpm....is this OK? Any problems / restrictions etc ?@?