My translation of the relevant line (beginning with "Location:") --
From a location in the footpath outside of No. 6 Divert Road, head south across the road to a location in the footpath outside of No 13 Divert Road.
The above is just a definition of the beginning and ending locations of some civil engineering works that are being carried out on behalf of Openreach.
F/W == foot way (a.k.a. footpath)
C/W == carriage way (a.k.a road)
O/S == outside
S == south
As for the reason for the excavation, I do not know. If you would take a few pictures of the relevant locations in Divert Road, Gourock, (including any of Beattie Bellman's cabinets) and then make them accessible for us to view we may be able to give you some idea of the purpose.