So out I went with the camera. Yes it was a mouse, not a House mouse, but a Harvest mouse. A beautiful little creature.
Harvest mice are extremely active climbers and feed in the stalk zone of long grasses and reeds, particularly around dusk and dawn. Their hearing is acute and they will react sharply; they either freeze or drop into cover in response to rustling sounds up to 7m away.
They eat a mixture of seeds, berries and insects, although moss, roots and fungi may also be taken. Harvest mice sometimes take grain from cereal heads.
Records of this species are taken because they are thought to have become much scarcer in recent years and they require conservation plans to reverse the decline. Changes in habitat management and agricultural methods are thought to be the main cause for the loss of populations from certain areas.