Natsudaidai or Natsumikan Orange
This is the Japanese summer orange or kawano natsu daidai.
Natsudaidai it is probably a hybrid between sour orange or pummelo and mandarin.
The flavour is bitter and sweet together and refreshingly pleasant not unlike seville orange marmalade.
It is almost the size of a grapefruit when ripe with a yellowish orange rind and when fully matured on the tree taste delicious.
One to watch for hardiness trials as this selection came from a tree growing outside in London for over twenty years producing hundreds of fruits and growing up to the roof of the house.