Surveys with submillimetre telescopes are revealing large numbers of gravitationally lensed high-redshift sources. I describe how, in practice, these lensed systems could be simultaneously used to estimate the values of cosmological parameters, test models for the evolution of the distribution of dark-matter halos and investigate the properties of the source population. Even the existing sample of lenses found with the Hershcel Space Observatory is enough to formally rule out the standard models of the evolving population of dark-matter halos, with the likely explanation a combination of baryon physics and the perturbation by infalling baryons of the density distribution of dark matter at the centres of the halos. Independently of the evolution of the halos, observations of a sample of 100 lensed systems would be enough to estimate Omega_Lambda with a precision of 5% and observations of 1000 lenses would be enough to estimate w, the parameter in the equation-of-state of dark energy, with a precision similar to that obtained from the Planck observations of the cosmic microwave background. While the fraction of submillimetre sources that are lensed depends weakly on the specific halo mass function that is used in the model, it depends very strongly on the evolution of the submillimetre luminosity function of the source population. Therefore measurements of the lensing fraction could be used to investigate galaxy evolution in a way that is independent of the properties of the intervening halos.
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