So here’s our very final clue, a blow-by-blow description from author Tommy Locke himself. It’s described in the direction that you’d actually ride the route.
Here’s what Lockey has to say: “Basically the bridleway singletrack dropping into the valley gradually doubles back Left through high-sided heath and enters tree cover. Just afterwards, before it tips steeply in the valley, just down the bank to the right lies an old forked oak tree, around the back of which, right at the base, and properly buried in the earth is the bottle and a dog eared copy of Escort. Happy hunting!”
The grid reference is: GR180/ST144391