Update...

Following my submission of a landlord's expression of interest form* and a wayleave, we had a visit from a Kelly team to assess the work needed to make the connection. It quickly became clear that the ducts put in 7 or 8 years ago were not as shown on the OR plan. About two weeks later an OR team was looking around our local area, but not at our building when I saw them. When I got back later on it was apparent that someone had been digging around in the footpath gravel where the ducts would be as per plan, so I infer that OR were following in Kelly's footsteps.

In the meantime I've managed to get hold of those involved in installing the ducts and telephone cable, and I've established that the duct ends short of the common entry doorway, and the cable comes in under the door sill (a large dressed granite block); it's then buried in the internal wall beside the doorway.

Still waiting to hear what OR come up with...

*http://www.superfast-openreach.co.uk/landlord/ although not stated, this applies to management companies of owner-occupied flats as well as to landlords of rented flats. There is a separate form for residents, but that won't achieve anything until the landlord's application has been accepted.