You will first need to complete the nearby Adventure Lab called Avery Hill Parkrun +.
The initial coordinates are for the first point of the Adventure Lab but there is no physical cache at this location.
Whilst completing the Adventure lab you will have been given values to substitute for the letters A to E in the following formula.
N 51° 2D.BDA E 000° 0C.EA(C-C)
You will need to bring your own pen to sign this magnetic cache, initials only prefered please.