I agree with Mario's post #2, get an ISA Arborist to come out and inspect the tree, your pics are great but its not possible to give more than vague suggestions without actually being on site in the case of such a normally resiliant and tough species.
No tree is impervious to the pressures of its environment not even Ginkgo, I would suspect as others have suggested the impact of alterations around the roots in the last 20-30yrs may be the cause.
Try here
ISA England Certified Arborists for a list of certified Arborists.