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.
I followed the drive gently uphill for three or four hundred yards, then took a short bridleway on the right that quickly brought me to the Ringshall to Northchurch road. The bridleway continued on the other side of the road, where I soon kept left at a bridleway junction. The bridleway took me to the drive to Northchurch Farm, where I turned left and took a path that went through a pedestrian gate and passed the farm. Just before a gate I kept left and then continued across what I think is a Polo field (another path went round two sides of this). On the far side, I went a few yards left and turned right, continuing between a hedge and a wooden fence on my right. Over a stile, the path continued down a track which soon turned left and then right, carrying on for another straight half mile, beside a huge field on my left. On finally reaching a field corner, the path continued half-left across a meadow to reach Coldharbour Farm.
The drive from "Gorseside's"
The short path after I turned right from the drive
The path across the road, heading to Northchurch Farm
The path across the road, heading to Northchurch Farm
The path passing Northchurch farm
The path from Northchurch farm (I think this is a polo pitch
The path from Northchurch farm
The path continuing towards Coldharbour Farm
The path continuing towards Coldharbour Farm
Approaching Coldharbour Farm
Just beyond Coldharbour Farm the Ashridge Estate Boundary Trail turns right, but I went straight on (this is how I normally shorten this route), following a bridleway along a roughly-surfaced drive, that curved slightly left as it descended gently through the trees of Berkhamstead Common. Just past Woodyard Cottages I reached the corner of a large pasture (there are sometimes a few cattle in it) surrounded by trees. Rather than take a footpath that runs through part of this pasture, I followed a roughly parallel path just inside the trees. Near another corner of the pasture, I turned right (at an unsigned junction) through a small patch of bracken and followed the footpath through the trees until I reached a track, where I turned right and fairly soon reached a drive as it approached Ashridge House.
The drive from Coldharbour Farm
The start of the path from Woodyard Cottages (I took the path just inside the wood, rather than the one across part of the enormous pasture)
Further along the same path
The path after I turned right
The track after I turned right, heading towards Ashridge House