When it cools down a bit, the simplest thing to do is to identify the incoming cable and the cable to the master socket and disconnect/ cut the rest. Phone wiring is extremely simple, it just uses 2 wires that are not polarity conscious.
the extension cables are owners problem openreach would probably just disconnect the extensions and leave the main connection to master socket. extensions should not be connected to the junction box although many people did that for convenience when setting up phone extensions this works fine but the star wiring is a killer for broadband
Just to close this. Openreach came and did as you said they would, disconnecting all the secondary sockets, other than the one they converted to the main socket replacing the one in the loft. Speed is now much better.
So thanks to those who advised and helped me sort this out.