  We present detailed model fits to observations of the disc around the Herbig
Ae star HD 163296. This well-studied object has an age of ~ 4 Myr, with
evidence of a circumstellar disc extending out to ~ 540AU. We use the radiation
thermo-chemical disc code ProDiMo to model the gas and dust in the
circumstellar disc of HD 163296, and attempt to determine the disc properties
by fitting to observational line and continuum data. These include new
Herschel/PACS observations obtained as part of the open-time key program GASPS
(Gas in Protoplanetary Systems), consisting of a detection of the [OI]63mic
line and upper limits for several other far infrared lines. We complement this
with continuum data and ground-based observations of the 12CO 3-2, 2-1 and 13CO
J=1-0 line transitions, as well as the H2 S(1) transition. We explore the
effects of stellar ultraviolet variability and dust settling on the line
emission, and on the derived disc properties. Our fitting efforts lead to
derived gas/dust ratios in the range 9-100, depending on the assumptions made.
We note that the line fluxes are sensitive in general to the degree of dust
settling in the disc, with an increase in line flux for settled models. This is
most pronounced in lines which are formed in the warm gas in the inner disc,
but the low excitation molecular lines are also affected. This has serious
implications for attempts to derive the disc gas mass from line observations.
We derive fractional PAH abundances between 0.007 and 0.04 relative to ISM
levels. Using a stellar and UV excess input spectrum based on a detailed
analysis of observations, we find that the all observations are consistent with
the previously assumed disc geometry.
