The large scale morphology and finer sub-structure within a slowly propagating traveling ionospheric disturbance (TID) are studied using wide band trans-ionospheric radio observations with the LOw Frequency ARray (LOFAR: van Haarlem, et al., 2013). The observations were made under geomagnetically quiet conditions, between 0400-0800 UT on 7 January 2019, over the UK. In combination with ionograms and Global Navigation Satellite System (GNSS) Total Electron Content (TEC) anomaly data we estimate the TID velocity to ~65ms^-1, in a NorthWesterly direction. Clearly defined substructures with oscillation periods of ~300 seconds were identified within the LOFAR observations of the TID, corresponding to scale sizes of ~ 20 km. At the geometries and observing wavelengths involved, the Fresnel scale is between 3-4 km, hence these substructures contribute significant refractive scattering to the received LOFAR signal. The refractive scattering is strongly coherent across the LOFAR bandwidth used here (25-64 MHz). The size of these structures distinguishes them from previously identified ionospheric scintillation with LOFAR in Fallows et al., 2020 where the scale sizes of the plasma structure varied from ~500 m – 5 km.