Thanks for all the input.
The exchange is Chipping Sodbury; Cabinet 13 (ECI). The modem/router is a BT Smart Hub. I have had similar results with a BT HH5A.
As for line length; it's possible it could go all around the houses or could be Ali. I know nothing about old Ali lines but I have heard of them; where they popular in the 70's? I think from memory my house was built 1975.