  We perform a detailed modelling of the post-outburst surface emission of the
low magnetic field magnetar SGR 0418+5729. The dipolar magnetic field of this
source, B=6x10^12 G estimated from its spin-down rate, is in the observed range
of magnetic fields for normal pulsars. The source is further characterized by a
high pulse fraction and a single-peak profile. Using synthetic temperature
distribution profiles, and fully accounting for the general-relativistic
effects of light deflection and gravitational redshift, we generate synthetic
X-ray spectra and pulse profiles that we fit to the observations. We find that
asymmetric and symmetric surface temperature distributions can reproduce
equally well the observed pulse profiles and spectra of SGR 0418. Nonetheless,
the modelling allows us to place constraints on the system geometry (i.e. the
angles $\psi$ and $\xi$ that the rotation axis makes with the line of sight and
the dipolar axis, respectively), as well as on the spot size and temperature
contrast on the neutron star surface. After performing an analysis iterating
between the pulse profile and spectra, as done in similar previous works, we
further employed, for the first time in this context, a Markov-Chain
Monte-Carlo approach to extract constraints on the model parameters from the
pulse profiles and spectra, simultaneously. We find that, to reproduce the
observed spectrum and flux modulation: (a) the angles must be restricted to
$65\deg < \psi+\xi < 125\deg$ or $235\deg < \psi+\xi <295\deg$; (b) the
temperature contrast between the poles and the equator must be at least a
factor of $\sim6$, and (c) the size of the hottest region ranges between
0.2-0.7 km (including uncertainties on the source distance). Last, we interpret
our findings within the context of internal and external heating models.
