The Mam Tor landslide (Derbyshire, England) slips downhill at up to 0.5 m a−1, and offers an excellent opportunity to study time-dependent creep in mudstones under in situ conditions. Annual surveys show that creep rates increase following heavy rainfall, but to establish detailed correlation between creep and pore water pressure required spatially and temporally higher resolution monitoring. We acquired 3 years of such data, at 3 h intervals, from wire creep meters, borehole piezometers and rainfall monitoring, showing that creep is strongly focused into the winter months and correlates well with pore water pressure. Summer grass and fern growth strongly influence rainfall infiltration, recycling much of the rainfall back to the atmosphere, explaining the seasonal variability of creep. The dependence of creep rate on pore pressure can be recast as a rheological model relating shear strain rate to shear stress using the friction angle for the creeping mudstone to link effective normal stress to shear stress. The creep is very non-linear, following a power law with a stress exponent of 48. This flow law may apply also to shear failure in poorly consolidated mudrocks under high pore pressure conditions; for example, within some landslides, subglacial sediments, tectonic fault zones, tectonic mélanges and accretionary complexes.