Microtremor networks have been instituted by the CEGB around the potential sites of PWR nuclear reactors at Hinkley Point in Somerset, Dungeness in Kent and at two locations in north Wales, as part of a programme for the assessment of seismic hazard. Critical attention has been paid to the quality of the data obtained and advantage has been taken of recently developed software to explore the confidence which can be attached to the findings of the networks. During the periods of operation of the networks eight microearthquakes have been recorded within 60 km of Hinkley Point and 22 close to the north Wales sites, but none within 60 km of Dungeness. Magnitudes range from 2.9 ML down to -0.5ML. Well constrained hypocentre locations indicate that mid- and lower crustal depths of origin are most common in the first two areas. Focal mechanism solutions for these events suggest that ‘mixed mode faulting’ characterizes current tectonic behaviour in both areas. These findings are exemplified by results from the Hinkley Point network. The direct and indirect implications for hazard evaluation procedures are discussed.