Considering the timing and the length of stay of the Cornish Great Spotted Cuckoo it might be a good idea to check the local emerging broods of both Carrion Crow and Black-billed Magpie, it's true that they usually breed as a pair and only a single bird was seen (as far as we know) but it only needs a fertilized female. The Cornish bird appeared to be in good health and finding lots of large caterpillars, occasionally the Cuckoo will parasitize the nests of Carrion Crow if a Black-billed Magpie is not available at the right laying period. (now that would be Bird News).