Cables go in four directions.
I've pondered this overnight and I think I have the answer.
My guess is that the stay is not there to resist the pull of the cables. It's there to support the pole when a ladder is placed against it. You lean the ladder against the side where the stay is.
If that's so, a new stay at 180 degrees to the old one might be a good solution. But I'm now somewhat more cautious, thinking that the safety of a BT "engineer" might be at stake.