I have some evidence that Hornet Moth may have been breeding at a couple of sites in Bath.  On the University campus, there are a few clumps of well-established hybrid poplars.  Most display exit holes, in some cases quite a considerable number (as shown below).  I managed to extract the cocoon shown below from one of these holes.

There is no sign of the moth, yet.  I have tried the pheromone lure on a couple of occasions (though on neither attempt was the weather ideal) and have looked for exuviae indicating an emergence this year, without finding any.  A careful look at the holes shows that many have been made in bark which has subsequently split away from the trunk to a slight degree, suggesting that they are a few years old and that the moth may not still be in residence.

I have since found a further single emergence hole and cocoon on the riverside in central Bath, in poplars just downstream from Churchill Bridge.