  We present deep submillimetre photometry for 14 galaxies at z=0.5 that are
hosts of type 1a supernovae, with the aim of examining the evolution of dust
mass and extinction in normal galaxies. We combine these results with our
previous observations of 17 z~0.5 SN1a hosts to look for any evolution in the
dust content of normal galaxies between z=0 and z=0.5. The average observed
frame 850micron flux of SN1a hosts in the full sample, excluding 2 bright
individually detected objects, is 0.44 +/- 0.22 mJy. This flux level is
consistent with there being little or no evolution in the dust content, or
optical extinction, of normal galaxies from z=0 to z=0.5. One galaxy, the host
of SN1996cf, is detected individually, and we also present a deep HST STIS
image for this object. It appears to be an edge on disk system, similar to the
submm bright host of SN1997ey. We thus examine the dust properties of these and
one other individually detected object. 450-to-850 micron flux ratios and
limits suggest that the dust in the two brightest submm sources, SN1996cf and
SN1997ey, is cold, T ~20K, implying that they contain a substantial mass of
dust ~10^9 Msun. The presence of two bright (F850 >7mJy) submm sources at z~0.5
in a sample of ostensibly normal galaxies is surprising, and has important
implications. It supports the idea that a substantial part of the Cosmic
Infrared Background (CIB) may be produced at z<1, while also suggesting that
'foreground' objects such as these may be a significant 'contaminant' in submm
surveys. Finally, we examine the overall submm luminosity distribution at z=0.5
implied by our results, and conclude that either there is substantial evolution
in the submm luminosity function from z=0 to 0.5, or our submm detected sources
are somehow not representative of the bulk of galaxies at this redshift.
