Don't know, it was in a crack in an old yew tree.
The clue is in the construction, bit out of touch nowadays, but a domed nest in a crack in a wall made with moss would suggest a Wren but the twigs look a bit too strong for such a small bird. Yew trees give a similar shaded environment to old wall sites. Seem to be a few sites in internet that give some info, all that I knew came from old farm workers, mostly long forgotten.