The purpose of the present work is to review some basic numerical homogeneisation techniques for the simulation of multiscale materials and to introduce an error control strategy at the local level. This error control uses an a posteriori error estimate built on a local problem coupling different representative volume elements. It introduces a weakly coupled adjoint problem to be solved say by a direct Schur complement method. Mortar element techniques as introduced in domain decomposition techniques are used to couple in a weak and cheap form the different representative elements in the error analysis. The strategy is numerically assessed on a model two dimensional problem.