The public debate about GM crops that took place in Britain in 2002-03 was an unprecedented experiment for the UK in public engagement. The Government, which sponsored the exercise, made a pledge: to take into account lay perspectives, as well as the views of its expert advisers, when making a decision about the possible commercialisation of this controversial technology. As officially accredited but independent evaluators of the debate, we were given unique behind-the-scenes access to the organisation and execution of the debate process. In this paper we report on our findings, which reveal a debate process that, whilst innovative, suffered from a number of important flaws: in terms of both design and implementation. These shortcomings in part reflect constraints on time and budget, and the context of political pressure in which the debate took place. However, they also demonstrated a lack of design knowledge and availability of appropriate expertise: matters with an important bearing on future plans for such initiatives.