The Lagrangian tropospheric ozone model ELMO-v2 (Edinburgh Lancaster Model for Ozone) is applied to the intense 2003 ozone episode in SE England. When using model parameters representative of typical episodes, ELMO-v2 was found to underestimate ozone levels substantially during peak ozone days, but, by increasing three parameters (temperature, biogenic and anthropogenic VOC emission rates) to levels close to those observed, good agreement between modelled and observed ozone was achieved. Using attribution techniques possible with a Lagrangian model, the episode was divided into five phases with each exhibiting different geographical origins for ozone precursor emissions. Anthropogenic VOCs, primarily of European (non-UK) origin, made the biggest contribution to modelled ozone levels. European biogenic VOC emissions significantly contributed to ozone levels on some days, whereas the contribution from UK biogenic VOC emissions was comparatively small throughout. The VOC:NOx ratio was also shown to change during the episode, with high ozone days being less VOC-sensitive. The implications of both variable NOx/VOC sensitivity and the possibility of more frequent heatwaves due to climate change need to be taken into account in planning effective future emissions reductions to control ground level ozone in the UK.