But doesn't a typical Wayleave allow originally PO Telephones, now BT OR, to access the land to maintain the line/s?
I don't have the original Wayleave Consent to hand, but here is an extract from a Land Registry title certificate:
A Wayleave Consent dated 8 July 1980 affecting the land in this title and other land in favour of the Post Office relates to the laying of telegraph lines and ancillary equipment on the land on the eastern side of [street name] which includes land adjacent to the south western boundary of the land in this title. The said Consent contains the following restrictive condition:-
"The Grantor shall not use any part of the Grantor's land in such a way as to make the Post Office telegraphic line difficult of access provided that a Grantor shall not be liable for any breach of this Condition after he has ceased to have any interest in the Grantor's land."
Wayleaves will typically provide for access for maintenance and similar routine activities. However, the existence of the grantee's right to maintain does not necessarily put the entire burden of preventative maintenance on the grantee.
It's expensive to go round trimming trees proactively. With electricity cables, there is a safety of life concern if the lines are touched or, worse still, brought down - even if the circuit breakers open, there might still be hazardous voltages on the cables until they are properly earthed. This provides some justification for routine tree trimming activities by the electricity companies. Conversely, a broken telephone cable is little more than a nuisance assuming that the cable falls safely away from any thoroughfare, so it is arguably open to Openreach to adopt a reactive strategy in respect of tree damage to overhead line plant.
Whether the landowner is responsible for paying for damage to overhead cables is likely to depend on any relevant conditions in the wayleave (if any), then on the law of negligence.