Hobbies / Books   (Gardening, walking, books, films, theatre etc.)

mollie m,
One of the fastest growing trees in England is the Alder, but the tree produces catkins and I have never seen one of these trees grow at a rate of 15ft per year.
I would also go with PeterP's suggestion (Elderflower or a type of Elder). Type in photographs of Elder trees or Sambucus and examine some of the details to see if any are the same as your tree.

By the way, your mystery of how it got there, isn't difficult to solve as berries are eaten by some birds and some small animals but the seeds aren't digested. These seeds, if swallowed, then are later excreted in their droppings and possibly some distance away.