Horizontal closed-loop ground collectors for ground source heat pumps are located within the soil and the top of the underlying superficial deposits. Estimating thermal properties for this zone is difficult as it is heterogeneous and is subject to seasonal water content variations. Soil thermal diffusivity values have been calculated at 56 sites using temperature data from UK Met Office weather stations. The technique utilizes the decrease in amplitude and increase in phase shift with depth of a transmitted heat pulse in the ground, the magnitudes of which are determined by thermal diffusivity. The weather stations are located throughout Great Britain and incorporate different soil types. The apparent thermal diffusivities derived from seasonal temperature cycles spanning several years generate seasonally averaged site-specific estimates that can be considered alongside diffusivity values determined in the laboratory or obtained by point measurements using field needle probes. Associated thermal conductivities have been estimated from the thermal diffusivities from knowledge of soil texture. Median thermal conductivities for the sand, loam and clay soil types have been estimated as 1.56, 1.15 and 1.81 W m−1 K−1 respectively with corresponding thermal diffusivities of 0.9961 × 10−6, 0.7173 × 10−6 and 1.0295 × 10−6 m2 s−1 respectively.