I've no idea as to the quality of that particular cable - it "looks" well made. The fact it is a round cable rather than flat probably means it is twisted pair which is a good thing. The braided screening actually won't do very much at all. For cable shielding to work effectively it really needs earthing at each end. That said, it's not going to make it worse 
There is such a big variance in the quality of these RJ11-RJ11 cables as there is no official standard as there is for telephony and data cabling.
As you say - give it a go and take it back if it performs badly 
Thank you for being so helpful.
Is it strange that I would want to run this permanently from the test socket ?
O2 are running a 24 hr test on the mastersocket (as I said, now isolated from the rest of the dodgy sockets in this house) - 3 dropouts in 24hrs on the previous test socket test, 6dB noise level. I can accept this if bandwidth is better than if noise level raised.
Is there any reason why the mastersocket (mm away from the test socket) would show lower performance than test socket (if isolated from the rest of the house) ?