In the east they are in good numbers up about as far as Lincoln, after that they get progressively thinner on the ground until you reach the Humber, where they stop. None north of the Humber in East Yorkshire. Further west I do not know too much about but there are occasional sightings in South and West Yorkshire. Further north there are muntjac in the North East, Co Durham I think, which it would be a good guess to say did not arrive naturally.