  The statistics of large-separation gravitational lensing are a powerful tool
to probe mass distributions on the scale of galaxy clusters. In this paper we
refine the analysis of our survey for large-separation (>5'') lensed FIRST
quasars (Ofek et al. 2001) by estimating the magnification bias and the source
redshift distribution. Finding no large separation lens among 8000 likely
quasars in that sample, implies an upper bound on the lensed fraction of 3.7 x
10^-4 at 95% CL. From a published deep 1.4 GHz radio survey of the Hubble Deep
Field, and corresponding optical searches for faint quasars, we calculate a
lower limit to the `double flux magnification bias' affecting our
radio-optically selected sample, of B>1.1. From the four-colour information in
the SDSS Early Data Release, we calculate the photometric redshift distribution
of a sample of FIRST quasar candidates and compare it with the redshift
distribution from the FIRST Bright Quasar Survey. We find that the median
redshift of the quasars in our sample is about 1.4. With these new results, we
find that for all plausible cosmologies, the absence of lensed quasars in our
survey is consistent with a model based on an empirical, non-evolving, cluster
mass function, where clusters are represented by singular isothermal spheres.
On the other hand, comparison of our results to the lensing predictions of
published N-body-ray-tracing simulations (Wambsganss et al. 1995, 1998) rejects
the COBE normalised \Omega_{0}=1 CDM model at 99.9% confidence.
