  We present new optical and infrared photometry for a statistically complete
sample of seven 1.1 mm selected sources with accurate Submillimetre Array
coordinates. We determine photometric redshifts for four of the seven sources
of 4.47, 4.50, 1.49 and 0.64. Of the other three sources two are undetected at
optical wavelengths down to the limits of very deep Subaru and
Canada-France-Hawaii Telescope images ($\sim$27 mag AB, i band) and the
photometry of the remaining source is corrupted by a bright nearby galaxy. The
sources with the highest redshifts are at higher redshifts than all but one of
the $\sim$200 sources taken from the largest recent 850 $\mu$m surveys, which
may indicate that 1.1 mm surveys are more efficient at finding sources at very
high redshifts than 850 $\mu$m surveys. We investigate the evolution of the
number density with redshift of our sample using a banded $V_{e}/V_{a}$
analysis and find no evidence for a redshift cutoff, although the number of
sources is very small. We also perform the same analysis on a statistically
complete sample of 38 galaxies selected at 850$\mu$m from the GOODS-N field and
find evidence for a drop-off in the number density beyond $z\sim1$ and 2 for
hot and cold dust dominated SMGs respectively, confirming the earlier
conclusion of Wall, Pope & Scott. We also find strong evidence for the
existence of two differently evolving sub-populations separated in luminosity,
with a higher relative density of the high luminosity galaxies at higher
redshifts.
