  We present a source and lens reconstruction for the optical Einstein ring
gravitational lens system RXS J1131-1231. We resolve detail in the source,
which is the host galaxy of a $z=0.658$ quasar, down to a resolution of 0.045
arc seconds (this is the size of the smallest conclusively resolved structures,
rather than the pixel scale), using a Bayesian technique with a realistic model
for the prior information. The source reconstruction reveals a substantial
amount of complex structure in the host galaxy, which is $\sim$ 8 kpc in extent
and contains several bright compact substructures, with the quasar source
residing in one of these bright substructures. Additionally, we recover the
mass distribution of the lensing galaxy, assuming a simply-parameterised model,
using information from both the quasar images and the extended images. This
allows a direct comparison of the amount of information about the lens that is
provided by the quasar images in comparison to the extended images. In this
system, we find that the extended images provide significantly more information
about the lens than the quasar images alone, especially if we do not include
prior constraints on the central position of the lens.
