Thanks very much MrLuke - screenshots and all! Very helpful :)
Just want to add to anyone trying this game on the iphone, when you reach the end of the road and scabbers is sitting in front of a building waiting, the screen pans up from where he is to show you a white grate above a doorway. This grate should have a spell glow on it. You have to get close to Scabbers without making him run away, trying to get the grate above the doorway to peek into your screen, then keep tapping it so that you can do a spell - it will convert into a cage and trap Scabbers. If you get too close to him, he will run off, and the grate will no longer be active. You then have to chase him back to his position in the doorway and try again. It is a flaw in the game that was obviously overlooked! I found that creeping in bit by it from the right was the easiest manouver. I hope that helps!