The alley can be seen on this 18th century map, just north of Temple Church.
The overlays that highlight 17th century London features are approximate and derived from Wenceslaus Hollar’s maps:
Open location in Google Maps: 51.513714, -0.110558
Reading eastward along Fleet Street from Temple Bar @its left end, read sideways "Hercules Pillars A[lley]" is the 3rd running south.
Chart showing the number of references in each month of the diary’s entries.