I agree with JayG. When you decide to get your topsoil, choose a couple of places but go and look at their supply first before buying, feel it with your hands as well, and make sure it has no big hard lumps (some may be due to compaction so see if they break up ok). I see now what you mean about the patio. To raise the height of the whole lawn would take quite a long time because the soil will continue to compact. If you just raise it at the patio edge, it might look a bit funny? Just a thought for now - you could make a little slope from the grass to the flags by using a single length of wooden boarding. Looking at it length-on, it would look like a right-angled triangle and all it would need is something underneath to support it. The other option could be to make a slope out of cement. Either way, you would need to cut an inch or two off the edge of the lawn where it meets the patio.