  We present an analysis of deep WSRT observations of the HI in 33 nearby
early-type galaxies selected from a sample studied earlier at optical
wavelengths with the SAURON integral-field spectrograph. The sample covers both
field environments and the Virgo cluster. Our analysis shows that gas accretion
plays a role in the evolution of field early-type galaxies, but less so for
those in clusters. For detection limits of a few times 10^6 Msun, HI is
detected in about 2/3 of the field galaxies, while <10% of the Virgo objects
are detected. In about half of the detections, the HI forms a regularly
rotating disc or ring. All HI discs have counterparts of ionised gas and inner
HI discs are also detected in molecular gas. The cold ISM is dominated by
molecular gas (M_H2/M_HI ~ 10). We conclude that accretion of HI is common for
field early-type galaxies, but the amount of material involved is usually
small. Cluster galaxies appear not to accrete HI. The few galaxies with a
significant young sub-population all have inner gas discs, but for the
remaining galaxies there is no trend between stellar population and HI. Some
early-type galaxies are very gas rich, but only have an old population. The
stellar populations of field galaxies are typically younger than those in
Virgo. This is likely related to differences in accretion history. In about 50%
of the galaxies we detect a central continuum source. In many objects this
emission is from a low-luminosity AGN, in some it is consistent with the
observed star formation. Galaxies with HI in the central regions are more
likely detected in continuum. This is due to a higher probability for star
formation to occur in such galaxies and not to HI-related AGN fuelling.
(Abridged)
