  We present a new photometric catalogue of the Coma galaxy cluster in the
Johnson U- and V- bands. We cover an area of 3360arcmin2 of sky, to a depth of
V=20 mag in a 13 arcsec diameter aperture, and produce magnitudes for ~1400
extended objects in metric apertures from 8.8 to 26arcsec diameters. The mean
internal RMS scatter in the photometry is 0.014 mag in V, and 0.026 mag in U,
for V13 < 17 mag.
  We place new limits on the levels of scatter in the colour--magnitude
relation (CMR) in the Coma cluster, and investigate how the slope and scatter
of the CMR depends on galaxy morphology, luminosity and position within the
cluster. As expected, the lowest levels of scatter are found in the elliptical
galaxies, while the late type galaxies have the highest numbers of galaxies
bluewards of the CMR. We investigate whether the slope of the CMR is an
artifact of colour gradients within galaxies and, show that it persists when
the colours are measured within a diameter that scales with galaxy size.
Looking at the environmental dependence of the CMR, we find a trend of
systematically bluer galaxy colours with increasing projected cluster-centric
radius which we associate with a gradient in the mean galactic ages.
