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You need to make it clear to ntl that you do not own the neighbour's wall. Thus you could not give ntl permission, even if they had asked (and this was discussed with them prior to installation and refused), to route the cable over the wall. Ntl will very well know that routing the connection in such manner requires an easement (legal right) to be created. Without the easement ntl, are creating a problem.
It may well be that ntl did not have a connection point in the right place. That is there problem, not yours. Ntl should have either required their contruction team to attend to amend the pavement connection (that could well have delayed the installation by some weeks, but they didn't really have an option), or alternatively if the connection couldn't have been altered they should have refused to install (or at least required you to get the easement before installing).
Ntl have created the problem, which they could hav avoided, albeit at risk of upsetting you by not being able to install.
To correct it now, the only initial way, untill they dig up the pavement, may be to remove the wire, and thus cut you off. Again ntl's problem.
In my view, altough ntl (or their installers) tried to be helpful, they created the problem so they should absorb the cost, not you.