This is the best restaurant I have been to in Peru in 2008. It used to be, or seem to me, that all restaurants were this good – Peru was like France in that way – but now they seem to have declined. In any case, we have the Cebichería Marlin and it is in Huamachuco.
From the Plaza de Armas take the calle peatonal to the end, and turn left immediately. The Cebichería Marlin is halfway down this block on your right.