  In the centre of our galaxy lies a super-massive black hole, identified with
the radio source Sagittarius A*. This black hole has an estimated mass of
around 4 million solar masses. Although Sagittarius A* is quite dim in terms of
total radiated energy, having a luminosity that is a factor of $10^{10}$ lower
than its Eddington luminosity, there is now compelling evidence that this
source was far brighter in the past. Evidence derived from the detection of
reflected X-ray emission from the giant molecular clouds in the galactic centre
region. However, the interpretation of the reflected emission spectra cannot be
done correctly without detailed modelling of the reflection process. Attempts
to do so can lead to an incorrect interpretation of the data. In this paper we
present the results of a Monte Carlo simulation code we developed in order to
fully model the complex processes involved in the emerging reflection spectra.
The simulated spectra can be compared to real data in order to derive model
parameters and constrain the past activity of the black hole. In particular we
apply our code to observations of Sgr B2, in order to constrain the position
and density of the cloud and the incident luminosity of the central source. The
results of the
