This shows the approximate area where Jessamy and Alverstoke traced the balloon after reaching Watford.  From Watford, in the lower right-hand corner, they were told that the balloon had come down "somewhere between here and King's Langley."  They left Watford on presumably what is now the Hempstead Rd.   "At the end of the second mile" out of Watford they encountered the local people, coming back across the fields from the balloon landing site.  They directed Alverstoke and Jessamy to Monk's Farm, "at Clipperfield, as you might say."  Alverstoke's party proceeded to King's Langley, which we see just north of the M25.  Just as they enter the town, Curry the groom spies a signpost for "Clipperfield and Sarrat."   They turn left (although this is not indicated in the text) and proceed "for two difficult miles" along the poorly tended lane.  They then reach Clipperfield and receive direction from a stout woman that Monk's Farm "lay about a mile down the road, towards Buckshill."  Our modern map shows Bucks Hill as two words, with Sarrat just SW of it.  At the top of the map, roughly 2-3 miles from King's Langley, is Hemel Hempstead, where Alverstoke stayed at the Sun after Frederica arrived.  I've added arrows at the approximate spots where I think the balloon landed and Monk's Farm were located.