In this drawing, you can see the area in the map and it is not 120m wide along the trench: https://www.nationalgrid.com/document/357086/download. For scale, the grid squares are 1000m.
A 400kV trench construction swathe also includes the soil storage areas - subsoil and topsoil are separated for return afterwards, as well as clearance to the fencing (https://www.nationalgrid.com/document/357086/download).