Gentlefolk,
The famous Ezzer has relayed the then Ts & Cs which stated you are responsible for everything within your curtilage.
However if BT have a wayleave across your land, whether acknowledged or not, I very much doubt that they could hit you for ageing faults but possibly still for inadvertent or wilful damage such as allowing your rotovator to run amok.
Another useful question to ask might be if you are responsible for BT's plant within your curtilage, then you should have the opportunity to employ your own, usually vastly cheaper, contractor.
If we had a truly independent Openreach they might just possibly have a more reasonable culture.
Kind regards,
Walter