Why does my bread have a dip in the center?
Two possibilities:


Your dough overproofed, or
the oven wasn't hot enough. In this scenario, the dough rises and then collapses before it can set


I would guess it was overproofed. Your recipe seems fine, though with all bread recipes, steps like "Leave for 1-1.5 hours" are highly variable depending on ambient heat in the kitchen, humidity, et cetera, and you should really employ the finger poke test to be certain.

References


http://www.kingarthurflour.com/tips/bread-troubleshooting.html
http://baking911.com/learn/baked-goods/bread/problems-and-solutions/general