We consider the Dirichlet boundary–value problem for the Helmholtz equation in a non–locally perturbed half–plane. This problem models time–harmonic electromagnetic scattering by a one–dimensional infinite rough perfectly conducting surface; the same problem arises in acoustic scattering by a sound–soft surface. Chandler–Wilde and Zhang have suggested a radiation condition for this problem, a generalization of the Rayleigh expansion condition for diffraction gratings, and uniqueness of solution has been established. Recently, an integral equation formulation of the problem has also been proposed and, in the special case when the whole boundary is both Lyapunov and a small perturbation of a flat boundary, the unique solvability of this integral equation has been shown by Chandler–Wilde and Ross by operator perturbation arguments. In this paper we study the general case, with no limit on surface amplitudes or slopes, and show that the same integral equation has exactly one solution in the space of bounded and continuous functions for all wavenumbers. As an important corollary we prove that, for a variety of incident fields including the incident plane wave, the Dirichlet boundary–value problem for the scattered field has a unique solution.