  We generate mock galaxy catalogues for a grid of different cosmologies, using
rescaled N-body simulations in tandem with a semi-analytic model run using
consistent parameters. Because we predict the galaxy bias, rather than fitting
it as a nuisance parameter, we obtain an almost pure constraint on sigma_8 by
comparing the projected two-point correlation function we obtain to that from
the SDSS. A systematic error arises because different semi-analytic modelling
assumptions allow us to fit the r-band luminosity function equally well.
Combining our estimate of the error from this source with the statistical
error, we find sigma_8=0.97 +/- 0.06. We obtain consistent results if we use
galaxy samples with a different magnitude threshold, or if we select galaxies
by b_J-band rather than r-band luminosity and compare to data from the 2dFGRS.
Our estimate for sigma_8 is higher than that obtained for other analyses of
galaxy data alone, and we attempt to find the source of this difference. We
note that in any case, galaxy clustering data provide a very stringent
constraint on galaxy formation models.
