Following a series if DNFs I've checked the coords with a different GPS device and updated only slightly - original coords were taken with a Garmin Glo, new ones are the average of 3 readings taken with my android phone. There wasnt much difference, but the phone said the accuracy was between 6 and 16 meters, the tree cover and hills dont help. I've added more to the hint to try to help...without making it too easy
In response to questions recieved: The "Sky tree" is a tree on top of a nearby hill, where the soil has eroded leaving the tree balancing precariously on its root structure, exposing tunnels and hidey holes in the roots. Though when the forest in in leaf, you probably cant see it from this cache, and the kids have since informed me that they call it the "floating tree" not the sky tree...but the cach name stands because it sounds better than "floating tree almost view at certain times of the year"
This is a large wood extending to 37 hectares, regenerating naturally following gravel extraction in the 20th century. In mediaeval times it was wood pasture common where local people grazed animals, gathered fuel and fed their pigs on acorns. There is a car park on Heath Road, an easy access trail close by and a longer perimeter trail for horse-riders and cyclists.
the terrain in mardley Heath is really interesting, loads of muddy banks and gullies which the kids love. See if you can find the "sky tree" where the soil around the roofs has eroded making the tree look like it's growing above ground (in the sky). If you're lucky you will find frogs and if really lucky newts in the ponds around the north pit area
for more info goto: http://www.hertslink.org/cms/content/15488274/16532555/mardleyheath.pdf