  We present a detailed mm-wave and optical study of the gravitational lens
system B1933+503, discovered by Sykes et al.(1998) in the radio. This object is
probably the most complex lens system known, with 10 lensed components within a
radius of one arcsecond. It is potentially important as a probe of the Hubble
constant, although no optical counterpart has thus far been observed down to
I=24.2. We have obtained new sub-millimetre detections at 450, 850 and 1350
microns. We have also constrained the possible dust emission from the proposed
foreground lensing galaxy using a K-band adaptive optics image and CO(5-4)
measurements. A lensing model is constructed, taking the foreground elliptical
galaxy at z=0.755 as the lensing mass. From this we derive a scenario from
which to model the sub-millimetre emission. Several arguments then point to the
source in the B1933+503 system lying above a redshift of 2. We speculate that
unlensed relatives of this source may constitute a sizable fraction of the 850
micron source counts.
