A number of these beetles were in the top of a compost bin after it had been topped up with rotted horse manure.
No interactions present.
John, just wanted to query whether we can see the pronotum clearly enough to rule out prodromus?
For more on this species pair see the Watford Coleoptera Group:
Entomologist and biological recorder
I have just been looking at that web page and I think the suture referred to is visible in the beetles at 2 o'clock and 7 o'clock in the picture. The others are at the wrong angle to see. That means those 2 are
Looking through a sample of 7 beetles found in the top of one of the bin lids, 6 are sphacelatus and 1 prodromus. The latter is larger than most, but not convincingly all, of the others. Some of them have mites attached below, including attached to the legs.
I wouldn't bet my house on it, but I reckon you can make put the pale basal rim of the pronotum in three of the those in the picture, if you view it at maximum size.
Lat/Lng: 51.9, -2.1
OS grid ref: SO9324