Posted: 22/04/2014 at 12:31
Certainly mice as everyone else has said. Try and clear the greenhouse as much as possible to avoid hiding places and block entry points with those cheap metal brillo pads (mice do not chew them). [confession, my greenhouse is very messy with bubble wrap all over the place and a shelf unit leaning alarmingly to the right]
PS, keep a look out for vine weevil and slugs - I have found a few over the past few weeks in the greenhouse at dusk.