per cent over time scales of tens of minutes in the optical [@Wang00]. *(ii)* Substructures in the jet can form if the bulk Lorentz factor depends on the angle inside the jet [@Meszaros98]. As the emitting region evolves through this patchy-jet, the flux varies in intensity. *(iii)* Angular inhomogeneity of the relativistic ejecta can separate the emitting surface into different causally disconnected regions [patchy-shell; @Kumar00; @Nakar04]. These wiggles evolve within the emitting surface, which is expanding in time with the blast-wave deceleration, causing variability in the radiation. *(iv)* If the ejecta have a range of bulk Lorentz factors, the slower shells will catch up with the leading blast wave, once the fireball has been decelerated by the external medium. The refreshed shocks will boost the luminosity of the afterglow [@Rees98]. *(v)* The GRB engine could contribute to the variability at late times: as debris accretes onto the black hole in the period following the burst, its extended activity could heat the environment or produce new outflows, giving rise to a detectable component of emission which, like any accretion-powered source, would be variable [@Rees00]. Thus, the detection (and even the non-detection) of time variability within several minutes to a couple of 