I've finally managed to identify one of the fritillaries. This one's become addicted to the window sill in the dining-room-to-be, where he (and it is a he - the stripe-y things on the upperside of his wings gives it away) sits out most of the day, in the midst of the plaster dust and lime and general grunge. Every evening he has to be persuaded to leave; every lunchtime he's back (what do butterflies do in the mornings, I wonder?). Here he is (please ignore the dust. It'll be gone by the time you get here, honest):
He's a Silver Washed Fritillary. Pretty, huh?