Posted: 06/05/2013 at 18:38
How many slabs are we talking and how wide is the area in question? Do you have plans to reuse the slabs somewhere else in the garden? If not, you could offer them on freecycle. As for what's under them I would get rid of it. If a thing is worth doing it is worth doing well. You don't want to go to the expense of buying shrubs, preparing soil, and as Danny mentioned the issue of drainage etc only to watch them die!
Do you have access to a car? Builders bags half filled could be transported much more cheaply this way than the hiring of a skip. Or as my friend did...every week put a few pieces into her bin bag until there was none left. Again how much are we dealing with?
Little shrubs will hopefully become big shrubs and they will look to get sustainance from the soil and also use it as an anchor - this need will only increase with the age of the plants.