For Neutral Beam Injection at JET an ion source is required with a high monatomic species yield. It is now clear that the magnetic filter field arrangement used to enhance the monatomic species yield also produces non-uniformity in the ion source plasma density. This property was known from Langmuir probe array measurements taken during development of the ion source. Further experimental evidence is derived from the analysis of detailed spectroscopic and calonmetnc measurements of extracted beams on the Neutral Beam Test Bed Nonumformity of the ion source density results in reduced transmission of the extracted beam, with greater interception on beamline scraper elements. The maximum power injected to the JET plasma from the Octant 4 Neutral Beam Injector is often limited by excessive beam interception on beamline scraper elements and not by the limits of the electrical power supplies. A development programme was initiated to develop an ion source configuration to produce improved uniformity, without degrading the monatomic species yield. This was approached in two ways - 1 Development of a Monte-Carlo model of the ion source. 2 An expenmental programme of source modification on the Neutral Beam Test Bed. The Monte-Carlo model was used to design and predict the performance of a new filter field arrangement. It was anticipated that the new source configuration would give improved ion uniformity without significant degradation of the monatomic species yield When operated on the Test-Bed this ion source design showed a factor of 2 improvement in ion density uniformity, with the required monatomic species fraction for JET NBI Beams extracted from the new ion source have demonstrated improved beam transmission on the Test-Bed and a 33% reduction of beamline scraper power load on the Octant 4 NB Injector. This is estimated to give a 5-8% increase in the neutral power mjected into the JET plasma.