Ellie, the only way to permanently fill a large hole in the soil is to replace it with more soil. Piles of stuff that was once living, called organic matter, will decompose completely, disappearing into finer and finer compost until it eventually turns into the molecules from which it was composed of.

Grass clippings, tree trimmings, fall leaves, etc. will disappear in weeks to months, depending upon the material and time of year. The first to go will be the green stuff and the last will be bark.

If you mix half of the volume of soil needed with organic matter, the level of the hole will sink slowly as the organics decompose.

So.....there's no way out of it. You'll need to find a source of real top soil, fill dirt which is composed of the mineral particles (sand, silt, and clay) that must be the building blocks of any soil. Organic matter is used only as a minor amendment.

Thanks Rhizo_1. I sort of thought that would be the case, sorta hoped it wouldn't be :-). But you have explained it so well that it totally makes sense. Soil it is :-).