I tried to intervene once. Late night, walking home from tube, vaguely aware a couple on the other side of the street arguing - then realised, as I passed them, that they were grappling and he was kicking her. I hesitated, but decided to Do Something and ran over to their side of the road, asking her if she was OK.
He let go - well, they let go of each other, I couldn't quite see who had grabbed who first. But he was bigger and stronger. Became apparent they were both drunk.
I asked her again if I could do anything or call anyone but she insisted not. He'd shambled off down the street by now. I made a final attempt to persuade her to let me get some sort of help, and she said 'He's my husband, he's my curse' and walked off. (In a strong Irish accent - this may have been before Ireland legalised divorce, can't remember quite when that happened.)
Then 200 yards further down I heard shouting and realised they'd caught up with each other, but clearly saw this time she attacked him so I decided to leave them to it...