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 went straight on along the bridleway on the other side of the road, until I reached a drive. The old Ashridge Estate Boundary Trail went straight on here, but the 'Ashridge Estate Boundary Trail - South' now turns right along the drive, then turns left onto a bridleway (by a cottage surrounded by high hedges). It then turned right (now back on the route of the old Ashridge Estate Boundary Trail) onto another bridleway. This was initially very muddy, but then nicely firm underfoot (this and several other bridleways were resurfaced a few years ago). The bridleway ran straight and level through more woodland for about half a mile, before turning slightly right and gradually going a little uphill to emerge from the woods by the Bridgewater Monument.
The bridleway after crossing the road near the Tom's Hill car park
The path after turning right along a drive
The bridleway after turning left
The bridleway after turning right
Further along the same bridleway
Further along the same bridleway
Further along the same bridleway, approaching the Bridgewater Monument
The Bridgewater Monument
I followed the bridleway past the cafe and visitor centre, then took a bridleway going half-left back into the woods (as usual there was a waymark to guide me here). I went straight on at three or four path junctions (I saw a Tree Creeper somewhere along here, the first I've seen in many years), then went half-left along a wide and muddy footpath (there was no obvious path going straight on here). I ploughed my way along this path (actually it wasn't quite as muddy as I'd feared) for just over half a mile, until a point where another broad path crossed this path. A post here had several different waymarks around it, including two Ashridge Estate Boundary Trail ones, pointing left and right. I turned right (going left is part of the 'Ashridge Estate Boundary Trail - North') and soon came back to the car park where I'd started.
The start of the bridleway from the Bridgewater Monument
The bridleway from the Bridgewater Monument
The bridleway from the Bridgewater Monument
The footpath from the Bridgewater Monument, after going half-left
The footpath from the Bridgewater Monument
The footpath after I turned right at a path crossroads, to return to the car park where I'd started
I really enjoyed this walk, even though I've walked much of the route several times before in recent years and thought I might have got a bit bored with it, and even though I did see far more people about today than I usually do (I'd forgotten quite how busy Ashridge can get on sunny days at a weekend!). There was a bit more woodland walking than on some of my walks, but enough farmland sections to stop it ever becoming boring. There were no outstanding views, but the scenery was pleasantly attractive the whole way around. I always thought the old Ashridge Estate Boundary Trail was a good walk, so it's hardly surprising that I've enjoyed both the new shorter routes that are based on it.