  (abridged) We present panoramic u' and optical ground-based imaging
observations of a complete sample of low-redshift (0<z<0.4) galaxies selected
in the ultraviolet (UV) at 2000A using the balloon-borne FOCA instrument of
Milliard et al. This survey is highly sensitive to newly-formed massive stars,
and hence to actively star-forming galaxies. We use the new data to further
investigate the stellar population and star formation properties of this
sample. When combined with our earlier spectroscopic surveys, these new data
allow us to compare star-formation measures derived from aperture-corrected Ha
line fluxes, UV(2000A) and u'(3600A) continuum fluxes on a galaxy-by-galaxy
basis. As expected from our earlier studies, we find broad correlations between
the dust-corrected star-formation diagnostics, though the scatter is larger
than that from observational errors, with significant offsets from trends
expected according to simple star-formation history (SFH) models. Varying
metallicities and/or initial mass functions seem unable to explain the observed
discrepancies. We investigate the star-formation properties further by
modelling the observed spectroscopic and photometric properties of the galaxies
in our survey. Nearly half of the galaxies surveyed possess features that
appear incompatible with simple constant or smoothly declining SFHs, favouring
instead irregular or temporally-varying SFHs. We demonstrate how this can
reconcile the majority of our observations, and determine empirical corrections
to calculate intrinsic star formation rates from measures based on UV (or u')
observations alone. We discuss the broader implications of our finding that a
significant fraction of star-forming galaxies have complex SFHs, particularly
in the context of recent determinations of the cosmic SFH.
