Rockery advice please :)

From the top of the steps, so you can see into the planting pockets.
So, finally getting around to the garden after over 4 years of doing the house up. One of the first projects is a rockery. I asked friends for donations of plants and they were amazingly generous! A lot of the stuff is best suited to a rockery though I think, so I built one.
Got free stone from gumtree-result The bulk of it is made from grass sods lifted to make beds, piled upside down. This structure is quite open, they didn't all fit together perfectly. I've jumped on it to try and settle it, so it doesn't sink once it's all in place!
My soil is about 1ft of clayey topsoil on top of clay. Well drained it ain't! So I've got compost and gravel to fill the rockery with. But this somehow seems a bit wrong, if you used compost in pots, you'd chuck it and use fresh the next season, but obviously I'm not going to refill the rockery each year! Should I dig this mix into the soil below? Any other tips on rockeries or clay soil or anything else really most welcome, thanks