Here are some full body photos... I have to say that I was suspecting a cockerel with the colouring and hackle feathers, although I just thought I'd double check before he goes to the RSPCA (I don't think my current roo would be too chuffed with another boy in the flock).
ETA: I'm pretty sure that he's a Cuckoo Marans, since it's a very popular breed in my area.