I confess I don't understand the most recent posts!
However, thanks to this, and the beauty of Google (other search engines are available) I think I can summarise my legal position as this:
In Re Ellenborough Park, 
Evershed confirmed four things must be present in order for an easement to exist:
- there must be a dominant and servient tenement;
an easement must "accommodate" the dominant tenant (the use of the land in question must be "connected" to the use of the dominant land;
the dominant and servient owners must be different people; and
the right must be capable of being the subject matter of a grant*.
*this is because easements cannot be passed by possession, because they do not physically exist – they can only pass through grants
*the right must be a "right of utility and benefit", and not merely for recreation or amusement
Properties that derive no utility or benefit from the alleyway (as they cannot use the alley-way to enter and exit the properties) do not meet the conditions necessary to be the subject matter of a grant. For this reason an easement (prescriptive or otherwise) cannot be granted.
In our situation, it is the installation of the gates in the boundary fences of the B-properties that changes the position from 'recreation and amusement' (i.e. the ability to merely walk up and down the alley-way) to one of 'utility and benefit' (i.e. the ability to use the alley-way to enter and exit the properties). Therefore it is the date of the installation of the boundary gates (not the gate that we own) that determines whether the properties can claim a prescriptive easement.
Thanks for all your comments and help.