Thanks to Sue Marshall for another picture from the threatened Radford Bridge Road Allotments in Wollaton :
It’s a view of Mistletoe (Viscum album) growing on an apple tree. Apple is one of the most common hosts for this hemiparasite, which has green leaves so it can photosynthesise, but takes other nutrients from the host. The plant is also the source of both the common and scientific names of the Mistle Thrush (Turdus viscivorus), one of the main distributors of the plant’s berries.
There is an excellent comprehensive source of information about Mistletoe HERE.