  We present details of the discovery of XLSSJ022303.0-043622, a z=1.2 cluster
of galaxies. This cluster was identified from its X-ray properties and selected
as a z>1 candidate from its optical/near-IR characteristics in the XMM
Large-Scale Structure Survey (XMM-LSS). It is the most distant system
discovered in the survey to date. We present ground-based optical and near IR
observations of the system carried out as part of the XMM-LSS survey. The
cluster has a bolometric X-ray luminosity of 1.1 +/- 0.7 x 10^44 erg/s, fainter
than most other known z>1 X-ray selected clusters. In the optical it has a
remarkably compact core, with at least a dozen galaxies inside a 125 kpc radius
circle centred on the X-ray position. Most of the galaxies within the core, and
those spectroscopically confirmed to be cluster members, have stellar masses
similar to those of massive cluster galaxies at low redshift. They have colours
comparable to those of galaxies in other z>1 clusters, consistent with showing
little sign of strong ongoing star formation. The bulk of the star formation
within the galaxies appears to have ceased at least 1.5 Gyr before the observed
epoch. Our results are consistent with massive cluster galaxies forming at z>1
and passively evolving thereafter. We also show that the system is
straightforwardly identified in Spitzer/IRAC 3.6 and 4.5 micron data obtained
by the SWIRE survey emphasising the power and utility of joint XMM and Spitzer
searches for the most distant clusters.
