The Isokon building, Lawn Road Flats, in Belsize Park on Hampstead's lower slopes, is a remarkable building. The first modernist building in Britain to use reinforced concrete in domestic architecture, its construction demanded new building techniques. But the building was as remarkable for those who took up residence there as for the application of revolutionary building techniques. There were 32 Flats in all, and they became a haunt of some of the most prominent Soviet agents working against Britain in the 1930s and 40s, among them Arnold Deutsch, the controller of the group of Cambridge...