In this paper we present an integrated packet/flow level modelling approach for analysing flow throughputs and transfer times in IEEE 802.11 WLANs. It captures the statistical characteristics of the transmission of individual packets at the MAC layer and takes into account the system dynamics due to the initiation and completion of data flow transfers. In particular, at the flow level the system is modelled by a processor sharing type of queue, reflecting the IEEE 802.11 MAC design principle of distributing the transmission capacity fairly among the active flows. The integrated packet/flow level model is analytically tractable and yields a simple approximation for the throughput and flow transfer time. Extensive simulations show that the approximation is very accurate for a wide range of parameter settings. In addition, the simulation study confirms the attractive property following from our approximation that the expected flow transfer delay is insensitive to the flow size distribution (apart from its mean).