We present new geophysical observations of the Mull Dyke Swarm in the Southern North Sea. 2D and 3D reflection seismic and aeromagnetic data were used to map the dykes. The three main dyke groups recognised onshore (Cleveland, Blyth and Hawick-Acklington) are found to extend to varying distances into the North Sea, crossing a number of major crustal-scale fault zones and domain boundaries, with almost no re-orientation. The Blyth Dyke Group extends furthest, for a distance of 672km from the source on Mull. The seismic data shows extensive development of pit chain craters above the upper tips of these dykes, which can be approximately dated to the late Palaeocene from the ages of crater fills. Volumetric estimates are made of the intrusive volumes associated with each group, ranging from 90km3 to 202km3. These three main axes of intrusion probably formed in different intrusive events within a c. 1 million year period, from 59 to 58Ma, during magnetic chron C26R.