Specifically it is a Garden Dormouse - here are some facts:
The Name "Dormouse": "Dormouse" is a misnomer from the French word "dormir," to sleep. These nocturnal animals are not in the mouse family, Muridae. They are often found sleeping during the day, hence their name.
Habitat: Terrestrial. Broadleaf woodlands, sometimes also coniferous woods; parks and orchards.
Description: Reddish brown on the back and off-white on the belly, the dormouse’s tail is black at its base and white towards the much thicker tip. A black stripe runs from the side of the snout, around the eyes and behind the large ears. These dormice reach a foot in length, five inches of which is the tail. They weigh over four ounces.
Behaviour: This dormouse lives at low altitudes, preferably in sunny areas. Although the dormouse is a good climber, it more often moves about on the ground - usually only during the night. Generally solitary, it may also be found in small, very noisy groups. Its nest may be among stones on the ground or in natural hollows in trees. Dormice hibernate throughout the winter and mate upon waking. It feeds on fruit and small animals and often enters houses in search of food.
Not sure I like the bit about 'can reach a foot in length' !