A novel crystal growth apparatus which separates the source and growth regions of a vapour transport system into separate vertical furnaces connected by a horizontal transport passage has been developed. The transport passage incorporates a flow restrictor which regulates the mass flow and decouples it from the source-sink temperature difference. Growth takes place on a seed crystal past which the growth envelope is dynamically pumped to ensure that the transport is diffusionless. This "multi-tube" arrangement offers several advantages over conventional linear vapour transport systems. In addition to control of the mass transport, source and growth regions are thermally decoupled and there is noninvasive optical access to both source and crystal growth regions and for in situ vapour pressure monitoring. In this paper, the results of in situ vapour pressure measurements made during the growth of a boule of cadmium telluride are presented and a flow model, which accounts for the significant molecular component of the transport, is developed and used to calculate the corresponding mass flows. This system offers the potential to give improved control of the nucleation and subsequent growth of large single crystals of high-vapour pressure opto-electronic materials such as the II-VI compounds.