An eggshell, presumably dropped by a parent bird. The jagged edge indicates that it was not a predated egg.
No interactions present.
This egg isn't likely to have been layed by a Dunnock - as far as I know there are no recorded incidences of them laying spotted eggs (not this spotted anyway). The ground colour does not look particularly blue in the photographs, but as it has been described as blue my best guess is that it is a Blackbird egg.
Thanks Roy, with only a 40 year-old copy of The Ladybird Book of British Birds and their nests to go on I knew there was a fair chance I'd get this one wrong!
Lat/Lng: 50.5, -2.5
OS grid ref: SY6676