The Mercia Mudstone Group (MMG) crops out extensively across England and Wales and its thermal properties are required for the design of infrastructure such as ground source heating and cooling schemes and electrical cable conduits. Data from the literature and new data from a borehole core have been compiled to generate an updated range of thermal conductivities related to rock type and the lithostratigraphy. These indicate a total range in saturated vertical thermal conductivity of 1.67–3.24 W m−1 K−1, comprising 1.67–2.81 W m−1 K−1 for mudstones, 2.12–2.41 W m−1 K−1 for siltstones and 2.3–3.24 W m−1 K−1 for sandstones. These data are all from measurements on samples and there will be uncertainty when considering the thermal properties of the rock mass owing to micro- and macrostructural features. Geometric mean modelling of thermal conductivity based on mineralogy has overestimated the thermal conductivity. Correction factors for the modelled thermal conductivities have been calculated to allow a first estimate of MMG thermal conductivities when only mineralogical data are available. Measured thermal diffusivities from the borehole core were in the range of 0.63–3.07 × 10−6 m2 s−1 and are the first measured thermal diffusivities to be reported for the MMG.