The Josephson photoresponse of granular high-Tc superconductor films to pulsed far infrared laser radiation has been investigated in magnetic fields of up to 3T. Its value is strongly influenced by fields less than 50mT and shows a pronounced hysteresis here. At low bias current densities, jb < 10A/cm2, an applied field stimulates the photoresponse, while for jb > 100A/cm2 the photoresponse is depressed. All changes have occured by a field of B < 50mT: for higher fields the photoresponse remains constant. This dependence is interpreted to arise from micrometre sized grain boundary junctions with a strongly inhomogenous critical current distribution on a sub-nanometre scale.