In reply to a post by Malwaremike:

RobertoS, my extension is 24-yr-old four core cable from a BT-installed extension. I paired the two pairs in effect making a twin cable.

What colours have you paired, and is it "twisted" form or straight wires? Twenty-four years ago they wouldn't have been worried about broadband signal integrity .

BT has been using twisted pair for many a year, so if original BT cable most likely twisted pair.

The old cable is four-core and the cores run straight inside the sheath, not twisted. Don't remember which colours I paired but they are the same both ends (if that makes sense!) and they pick up the original two connections from the entry box.

The overhead cable from pole runs inside the roofspace to a black junction box on a purlin, then a grey two-core cable runs down into the hall to a small 65mm square socket box, probably is 40 yr old original. Front is secured by two screws and there is no test socket, just a couple of electrical components at the top and a row of push-in connectors down each side, my extension cable is connected direct into these.

Will try disconnecting the extension and plug the Hub direct into the socket again. Thanks everyone.

By doubling up you have reduced attenuation by perhaps 0.1dB, but increased ability for noise pickup that would knock out any gain I suspect.