Apparently, disclosure could a) harm UK international relations b) prevent civil servants from doing their job and c) unlawfully identify individuals and their personal information.
I've vigorously argued each point (see the link for details) - and I've asked for an internal review of the decision. If anyone has any further suggestions for how I can pursue this then please let me know.
One glimmer of hope is that the exemptions applied for a) and b) are both subject to the 'public interest test', i.e. they can be over-ridden if the benefits of disclosure are great enough.
PS. One of DfID's 'Departmental Strategic Objectives' (i.e. what Minsters judge them on) is to improve the 'portfolio quality index' - a measure of the proportion of projects evaluated as successful. This measure runs off the performance data in ARIES - surely this is a strong public interest argument for disclosure?