'Locating our network' or 'Altering our network' depending what you want to do.
Making contact and receiving sensible information from Openreach though is at least as (or even more) convoluted than communicating with VM so don't expect to get anything too useful or sensible out of them!
Rediffusion, like British Relay Wireless was a first generation multiwire "relay" service for off air reception. All these services ceased in the early 1980s.
Rediffusion was part of the group that eventually formed Granada TV, which now basically runs ITV. It is obselete abandoned equipment now, like all our BRW ducts in the road & cable round the eaves of our houses here. Neither Virgin nor Openreach is responsible for it, so I would contact your Local Authority safety team if action needs taking.
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If the offending pole is in a back garden on private land then I think I can hazard a guess on what the LA will say!
Even if you fully establish that the pole/line is totally defunct and not in use, you will presumably still have to get in touch with the freeholder of the flats as well to gain their permission to access their land to disconnect the cable at their end too?