Roger Hayter wrote:

You could try wiring this extension with proper twisted pair instead of
ordinary telephone internal wiring which isn't (twisted, that is).

It seems impossible to find an official GPO/BT/Openreach spec for CW1308
internal phone cable, but various manufacturers specify theat they use
different lay-lengths for the individual twisted pairs, e.g.

http://www.caledonian-cables.co.uk/Telephone/Indoor/CW1308.htm