We thought at first that this was a large dog, lurking in the undergrowth at Holkham NNR in Norfolk, until be realised that we were looking at a muntjac. They are so small that they can easily conceal themselves in a patch of brambles.
These miniature deer, first introduced at Woburn from China in 1901, have spread rapidly throughout much of England over the last three decades and are now well established as far north as Yorkshire, with small populations in Scotland. They can have a devastating effect on the ground flora of woodlands, grazing the flower buds of bluebells, orchids and most other woodland wild flowers, including rarities like the oxlip. There's an interesting article on the ecological effect of the deer population explosion of native and introduced deer in the April 2010 issue of the excellent British Wildlife magazine.