A neutron small-angle scattering study of the flux-line lattice in heavily twinned YBa2Cu3O7-x is presented. It is found that the diffraction signal disappears at temperatures well below: T-c, associated with a melting of the flux lattice. The shape of the melting line is consistent with both a Lindemann criterion and the scaling expected for a vortex-glass transition with the superconducting parameters from the three-dimensional XY model. The influence of twin planes on the structure of the vortex lattice and its melting is studied by applying; the field at different angles to the c axis. The results are compared with recent specific heat measurements on similar crystals.