Mystery Plant

This plant has been steadily growing in the un heated greenhouse since I first spotted it in the Autumn, with 3 tiny leaves. It has continued during the coldest of conditions and I have been content to leave it, just for amusement. At first I thought it might have been a variety of Poppy which can crop up anywhere in the garden, but it would not be growing strongly at this time of year.  I have left my feet in shot for the purposes of scale .  A complete mystery?