hmmm....big dips huh. I seem to remember Alan Titchmarsh (if that says thingymarsh you know who I mean don't you :-\) cutting like an X right through the middle of the dipp, gently slicing back the turf and peeling it back to reveal the soil beneath, forking the exposed dip gently, then adding some more topsoil to bring the dip up level, firming it well, otherwise it will only sink down a bit again, then relaying the turf , firm and water well and regularly. Does that seem about right anyone? If it was only a small divit, then I believe that you can rake in a mix of sharp sand and topsoil or compost and the grass with grow up through and level itself...if you kinda understand what I mean. Hope this is of some help. :D