Of all the things that you may expect to see while in Longfield, this probably isn't one of them!
There is a green cabinet with the number AB on near where you are standing and the cache can be found at:
N51 23. 3B-A 4B-4A 4A-B
E0 19. 2A-B B-A 3B-4A
You are looking for a 1.5ml pot.