  We study the disorder-induced localisation transition in a three-dimensional
network model that belongs to symmetry class C. The model represents
quasiparticle dynamics in a gapless spin-singlet superconductor without
time-reversal invariance. It is a special feature of network models with this
symmetry that the conductance and density of states can be expressed as
averages in a classical system of dense, interacting random walks. Using this
mapping, we present a more precise numerical study of critical behaviour at an
Anderson transition than has been possible previously in any context.
