To find this cache you would normally have to complete the Headley Saunter series of 3 caches, and the Mickleham Ramble series of 3 caches. Each of those caches contains information for the coordinates of this cache.
Alternatively, try some coordinates in the geocheck and see what happens.
The cache can be found somewhere on or near the Mickleham Ramble walk and is at
N51 16.(BxFxC/D)+3 W000 17.(AxFx(E-D))+15
Also nearby is an archived cache, but you can still find it and log it if you so desire!