Mystery (to me) flower

The garden next to us has a flowering cherry. Its close to the fence so we get the benefit of admiring it from ourĀ window.

I’ve notice recently another plant that seems to be growing up into the flowering cherry.

Its in flower now.


Does anyone know what it is?

UPDATE: the very helpful @josordoni @ailbhetweets and @theamplecook all tell me this is clematis montana, thank you for solving the mystery