If it's any size at all and if it were here in the U.S., it would be worth a gold mine... even at today's inflated prices.
Black walnut, especially is highly prized and an average tree of, say 35 to 40 feet in height with a diameter of 18 to 24 inches would be valued at close to $25,000 (U.S.) The lumber is used,mainly, for veneering since solid walnut is far to expensive from which to construct fine furniture or cabinetry.
Before you have it removed, check your Yellow Pages (or the British equivalent) for 'Hardwood Dealers'... they would gladly (again here in the U.S.) pay for the cost of removal just for the prize of being the first in line to bid for the tree.
Keep in mind, the value does decrease when the tree is small and if it has excessive knots and surface damage. For highest value, the tree should be measure, at about 5 feet above the ground, a diameter of at least 18 to 36 inches. The more diameter, the more value. There should be at least 10 to 20 feet of straight, sound trunk from that point to the first branching (which produces knots), again, for the highest value.
Best of luck with your treasure!