In the lower stratosphere the budget of ozone is controlled by a combination of transport processes and chemistry. A knowledge of the importance of these different terms is essential for understanding the causes of any long-term changes in ozone. Also, the balance between these terms may vary interannually and between the hemispheres - and these different cases all need to be examined. In general there is a lack of data in the SH high latitude lower stratosphere, though recent observations were provided by the APE-GAIA campaign in September-October 1999. In this campaign the M55 Geophysika sampled the lower stratosphere from around 55S-70S and from the tropopause up to 20 km. We will discuss diagnostics of the stratospheric O3 budget (including dynamical and chemical contributions) using the output from the SLIMCAT global 3D chemical transport model. The model was integrated in a number of experiments for the Antarctic winter/spring in 1999 and we will also compare output with APE-GAIA chemical observations. We have investigated the sensitivity of the model results to a range of parameters (e.g. model resolution, meteorological analyses used to force model) and these will be discussed.