Thanks for your replies.
I’ve seen ITT increases as a result of dirty fans and I understand the reasons behind this. In my mind the fan is essentially multiple aerofoils, which when dirty create more drag which in turn requires more energy to reach the desired N1, hence increased fuel burn.
What I’m really interested in is for a given N1, is the resultant thrust less?
When I think about this deeper, a dirty/contaminated wing creates more drag/less lift, to maintain level flight, in simple terms is counteracted by an increased AoA which in turn is balanced with an increased energy input in the form of a higher engine power setting. So in in the case of a dirty engine fan, this additional energy input to maintain target N1 is additional fuel, but again surely this results in less thrust... or are the aerodynamics involved in the fan far less susceptible to these kinds of losses and are therefore overall negligible?