Thanks for your suggestions.
This proved very fruitful. The first lecture covers a lot of this material and at the end Bhikkhu Bhodi recommends the following (in case any one else is interested):
Richard Gombrich “How Buddhism Began”
Uma Chakravarti “The Social Dimensions of Early Buddhism”
Wolfgang Schumann “The Historical Buddha”
Rupert Gethin “The Foundations of Buddhism”
Amazon reveals that some of these are hideously expensive (Gombrich is $150
) but it also turns up some interesting looking suggestions. Maybe it's time to smash the piggy bank!