There are a number of ways in which a review can be organised, ranging from a chronological presentation of work done (with some evaluation) to a conceptual framework to which individual contributions can be attached; or on another axis, from commercial application as the prime focus to academic viewpoint. I have chosen to emphasise a particular conceptual framework as being the most effective basis for the argument in an attempt to l~nd coherence to this particular presentation (rather than to claim unique validity). This approach has the advantage that it is then relevant to any point on the commercial-academic spectrum and can be used to situate and appraise a variety of applications and research studies of each kind. The review is 'British' in the sense that much of the research described was carried out in Britain, but inevitably there are some antecedents and follow-up research in other countries to which it is also appropriate to refer.