c128 said:...Do BT use external junction boxes for this sort of thing?
Yes, there is a BT type designed to join the older figure 8 drop wire to the newer external drop wire for the route from the JB into the house. They have connections designed ( including type of metals used ) to provide good joints to the metals used in the different drop wires.
If so, and I do this too, are they likely to reuse it should the drop wire need sorting?
The sticky point is that the linesman who replaces the drop wire may go "Jobsworth" and refuse to use your junction box as it will almost certainly not have the BT logo embossed on it.
If both of the above are yes, where can I get such a box?
Only from BT......
With an external junction box you will need to install a length of external grade cable between the junction box and the master socket.