Did you book a 'change point of entry' type service appointment from BT?
I'm somewhat uncertain as for the purpose of the appointment. I would not expect an ISP/CP to agree to book a speculative appointment . . . unless you have agreed to pay the charges.
A new pair provide will use a spare pair within the incoming service feed (assuming the spares are not defective) and not involve a new run from the DP . . . unless you have agreed to pay the cost of materials used.
Any competent technician, whilst working on those circuits, would most likely be mindful of removing that BT in favour of using gel-crimps.
I can still cancel the appointment.
I contacted BT via their online chat and told them that we were doing building works and that the wires looked to be problematic and I wanted them to look at them before carrying on.
They said they could send someone out to look at improving it but that it may be chargeable in line with
http://bt.custhelp.com/app/answers/detail/a_id/12439/~/engineer-charges , that they would tell me if it would be chargeable to do the work once they'd looked at it.
Is there another route I should go down with BT (change point of entry?) perhaps?
My logic with the other pairs idea is that if there are spare undamaged pairs in the service cable that it would be worth connecting a pair of these to the existing wire which comes across to the NTE. This would give me a "backup pair" should the pair that is in use currently develop a fault, without having to access the panel.
All the ways I can think of of maintaining access via hatches, shorter walls, etc have large implications for the works.
In the scheme of things, £130 to significantly reduce the likelihood of having these connections fail would be worth it. The whole job is going to be one massive wallet haemorrhage as it is...