This is the first time in over 16 months that i have been back on this forum having sold my Alfa GT V6 with a creaky clutch. i replaced everything twice except for the flywheel and the master cylinder, i was adamant it was the slave but this was changed three times.
Like many people on here have said its a case of spraying the bit where the metal rod goes into the bulk head, it can take time and it is difficult. I used the white grease stuff and it made the pedal much smoother and the creak was less noticeable.
But with the 159 be careful as the cruise control thingy is close to where you have been spraying.
I'm not sure what the cost would be to get it replaced, but give Lewis a call @ Alfa Tecnico and he might have seen this problem on a 159 before.