If you are considering walking this route yourself, please see my disclaimer. You may also like to see these notes about the maps and GPX files.
The bridleway immediately passed a bungalow on the right, and continued for several hundred yards between old hedgerows either side. It eventually emerged onto the top of Bison Hill, where I followed a path going half-right through the grass, descending slightly. At a path junction in the grass I turned right to reach a metal kissing-gate (another path was a short way to my left, close to a road), through which a path continued with the hillside soon sloping down to my left. I soon reached a faint fork where I kept left, the path curving slightly left and then to the right. After a few hundred yards there was a steep descent for a very few yards that brought me to a crossroads of sunken tracks worn into the hillside - these are old drovers' ways i.e. paths shepherds used for centuries to move their sheep from place to place.
The bridleway to Bison Hill
The bridleway to Bison Hill
Arriving at Bison Hill
Path on Bison Hill
Path on Bison Hill
Path on Bison Hill
Path on Bison Hill
I went straight on here, dropping more steeply downhill. I slowed right down and spent the next 30 minutes or so going very slowly as I was now looking for butterflies and orchids - I've included a few photos of what I found. At the bottom of the slope I followed the path as it curved right and passed just left of a large pylon. Soon the path was again a sunken track with an embankment on my left, gradually working its way back up the hillside. This was the main area I was expecting to see Orchids, but I was a week or two too early and only saw a few Common Spotted Orchid and a solitary small Pyramidal Orchid.
The old Drovers' Way descending Bison Hill
The old Drovers' Way descending Bison Hill
The old Drovers' Way slowly ascending Whipsnade Downs
The old Drovers' Way slowly ascending Whipsnade Downs