The Detection, location and quantification of gamma emitting radionuclides is an essential process in the management and decommissioning of nuclear sites. In many applications, gamma imaging is the best way to implement this process; it has superior spatial resolution to a manual survey and can operate at a distance from the target area, or in very high dose fields. In this paper we introduce a new imaging method for gamma radiation that uses a novel collimator design and reconstruction algorithm to overcome the limitations of simple single detector imagers. The performance of the system is characterised in the laboratory and demonstrated at a civil nuclear site. Compared to previous systems, the system is found to have better spatial and spectral resolution, faster scan times, a larger field of view and lower mass and size.