DOMDIEGO'S PROFILE

Please, can anyone help me? I'm really stuck in the game... i've picked the rat killing quest and i can't complete it. I kill all the rats in town, enter the storage house north, kill some more rats, confront the hamelin guy, get the flute, get out, kill all the rats again... and nothing. What do i have to do to complete this quest?
Many thx in advance.
Cheers.