Wondering why, I looked more closely at the fence he was sitting on and realised that the pair had built their nest on top of the orchard fence, between three posts.
Robin's aren't famous for choosing sensible locations for their nests but this one is particularly badly located. The weasel that was killing our chickens in the winter is still around and I've seen it running along the fence this week.
I don't think the contents of these beautifully-coloured eggs stand a chance. :-(