The Butterknowle Fault is a major normal fault of Dinantian age in northern England, bounding the Stainmore Basin and the Alston Block. This fault zone has been proposed as a source of deep geothermal energy; to facilitate the design of a geothermal project in the town of Bishop Auckland further investigation of its geometry was necessary and led to the present study. We show using three-dimensional modelling of a dense local gravity survey, combined with structural inversion, that this fault has a ramp-flat-ramp geometry, ~250 m of latest Carboniferous / Early Permian downthrow having occurred on a fault surface that is not a planar updip continuation of that which had accommodated the many kilometres of Dinantian extension. The gravity survey also reveals relatively low-density sediments in the hanging-wall of the Dinantian fault, interpreted as porous alluvial fan deposits, indicating that a favourable geothermal target indeed exists in the area. This study demonstrates the value of gravity data for elucidating geological structure, even in a well-studied region such as Britain, and highlights the need to verify published structural interpretations as future deep geothermal projects are designed. Future work of this type might be undertaken more expeditiously using microelectromechanical gravimeters.