The roof garden

      We were out walking the two terriers on a beautiful, sunny, Saturday morning when we spotted this succulent growing on a pig sty roof.
      We have no idea what the plant is called or why it's growing there but it is well established and is even hosting a Cotoneaster to further extend the roof garden, although the Cotoneaster looks as if the winter has got the better of it.
      Is this the beginnings of a roof eco-garden?