A detailed gradiometer survey was conducted over land adjacent to Junction 20 of the M25, near Watford, Hertfordshire (centred on NGR 507724, 200907). The project was commissioned by The Millbridge Group Plc on behalf of Moto Hospitality Ltd with the aim of establishing the presence, or otherwise, and nature of detectable archaeological features in support of a planning application for the development of the site as a new motorway service station. The site comprises two pasture fields located directly south-east of Junction 20 approximately 2 km west of Abbots Langley and 1.9 km south of Kings Langley, covering an area of 14.3 ha. The geophysical survey was undertaken between 18th and 22nd April 2016. The detailed gradiometer survey has demonstrated the presence of a number of anomalies of possible archaeological interest. The anomalies identified as potentially being of archaeological interest are primarily clusters of pit-like features. These features identified as such can be seen widespread across the southern field. It is difficult to be certain about their origins but an archaeological interpretation cannot be excluded with the currently available information. The northern field is dominated by strong ferrous responses likely from the construction of the M25. However, there are some faint linear and curvilinear trends extending across this field. It is possible that these trends relate to modern features. Additionally, this archaeological investigation has detected areas of increased magnetic response, areas of near surface geological change and evidence for historic cultivation.