Massachusetts Institute of Technology - "Redrawing the Map of Great Britain from a Network of Human Interactions
Using an anonymised database of 12 billion calls in one month, MIT found several distinct areas of the UK, based on where people are phoning. Locally they found that London extends beyond its boundaries to include most of Hertfordshire, while the bulk of Kent, Surrey and Sussex form another region. Most of the remainder of the Government's South-East region (Hants/Berks/Bucks/Oxon forms another telephone call region.
Being near London, Hartley falls in a border zone between London and Kent/Surrey/Sussex, where their influence overlaps.