Flutterby

This little beauty fell at my feet this morning when I opened the shed door.  There she sat, despite me telling her she was going to get run over by the lawn mower if she didn't move!  But she sat, ignoring me.  Maybe she was cold and needed to warm up before she could fly so I gently lifted her and placed her on a leaf in the sun.  Wouldn't you know, 20 minutes later her antennae were up and she was looking perkier and shortly after that she took flight and went on her way.
I am slightly flummoxed by her appearance, we've not seen this butterfly before in the garden!  Any idea of who she is??