  The well defined selection function of the XMM-LSS survey enables a
simultaneous modelling of the observed cluster number counts and of the
evolution of the L-T relation. We present results pertaining to the first 5
deg2 for a well controlled sample comprising 30 objects: they are compatible
with the WMAP3 parameter set along with cluster self-similar evolution.
Extending such a survey to 200 deg2 would (1) allow discriminating between the
major scenarios of the cluster L-T evolution and (2) provide a unique
self-sufficient determination of sigma8 and Gamma with an accuracy of ~ 5% and
10% respectively, when adding mass information from weak lensing and S-Z
observations.
