  We analyse a specific two dimensional mixed spin Heisenberg model with
exchange anisotropy, by means of high temperature expansions and Monte Carlo
simulations. The goal is to describe the magnetic properties of the compound
(NBu_{4})_{2}Mn_{2}[Cu(opba)]_{3}\cdot 6DMSO\cdot H_{2}O which exhibits a
ferromagnetic transition at $T_{c}=15K$. Extrapolating our analysis on the
basis of renormalisation group arguments, we find that this transition may
result from a very weak anisotropy effect.
