  We present the results of an unbiased radio search for gravitational lensing
events with image separations between 15 and 60 arcsec, which would be
associated with clusters of galaxies with masses >10^{13-14}M_{\sun}. A parent
population of 1023 extended radio sources stronger than 35 mJy with stellar
optical identifications was selected using the FIRST radio catalogue at 1.4 GHz
and the APM optical catalogue. The FIRST catalogue was then searched for
companions to the parent sources stronger than 7 mJy and with separation in the
range 15 to 60 arcsec. Higher resolution observations of the resulting 38 lens
candidates were made with the VLA at 1.4 GHz and 5 GHz, and with MERLIN at 5
GHz in order to test the lens hypothesis in each case. None of our targets was
found to be a gravitational lens system. These results provide the best current
constraint on the lensing rate for this angular scale, but improved
calculations of lensing rates from realistic simulations of the clustering of
matter on the relevant scales are required before cosmologically significant
constraints can be derived from this null result. We now have an efficient,
tested observational strategy with which it will be possible to make an
order-of-magnitude larger unbiased search in the near future.
