The Iapetus suture, arguably the most fundamental lineament of British and Irish structure, has been previously identified on the BIRPS WINCH deep seismic reflection profiles offshore as a NW-dipping feature. New depth-migrated interpretations of these and other reflection data show good correlation of structure for 100 km along strike in the northern Irish Sea towards magnetotelluric stations onshore in the north of England and Southern Uplands of Scotland. Three different methods of inverting data from these sites all reveal a thin NW-dipping slab of high conductivity. A contour map of the suture trace, dipping at 15–25°NW, can be constructed down to and through the Moho at 28 km depth. This map suggests crustal stretching below the Solway Basin. The ?mylonitic suture zone is characterized by good seismic reflection layering, high conductivity (presumably fluid-produced), and a steep increase of P-wave velocity over the few kilometres of its width. Crustal refraction experiments LISPB and CSSP provide useful velocities, but locally misleading structure, so that the combination of the seismic reflection and geoelectric methods may be the most powerful means of determining deep crustal structure with a good degree of both vertical and lateral resolution.