Goat Willow (Salex caprea)
These large yellow flowers are quite conspicuous now as, like the Blackthorn, the flowers come before the leaves.
This is a common tree which is just as well as the flowers are a major nectar source for our early insects, especially moths. In a cold spring easy to find nectar sources can be the difference between life and death for many insects.
When these are just buds they are known as pussy willow and are a silvery white colour. When the flowers fully emerge, however, the silver gives way to gold and they make a lovely sight.