You're probably right - these cases are very thin and very soft, so tend to form more than they cut. I think if I had a harder mandrel, something like delrin or copper, it'd work better.
However, as is usual when you tell someone your problems, I think I've found the solution. I'll make a tube of the right outside diameter and length, and that way I can use a fine sawblade to cut them down. I'm afraid the angle grinder idea might be overkill in a London flat! Does anyone know if I'm right as to length before I go mutilating 50 of them?