WP1.
The given co-ordinates will bring you near the south/west gate of the park. There is an anniversary bench.
The year on the plaque is: xDDx. The second and third digits are the same and this is D.
WP2.
Now you are going up the hill. You can see the Wembley Stadium from here.
There is a tree at the given co-ordinates with a plaque in loving memory of Lance J.E.Hamilton.
His date of birth is: Cx-xC-xxxx. The first and fourth digits are the same and this is C.
WP3.
Now down the hill. On the right you will see a lovely Victorian-style Gazebo which was placed here to mark a centenary of the park in 1995.
On the left you will find an Aviary. Check how tall is a cockatiel. This is E.
WP4.
The fourth co-ordinates will bring you to the map of the park. The Extension is ABx meters long.
Now you have all the numbers. The final can be found at:
N51 32.ABA W000 14.CDE
You are looking for the black bison tube.
Good luck!
Congratulations to Brucacher on the FTF