So, now 'genuine' parts are just as rubbish. Great.
Thanks to the dreadful design of the handbrake system, someone had completely over tightened the adjuster nut, causing it to spin at the bottom of the thread on the short cable.
All the adjustment takes place at the bottom end of the thread, so it's no wonder so many get wrecked. Why the cable couldn't be 10mm shorter I don't know.
So everything out of the car so remove the handbrake lever to get the nut off and replace the short cable.
'Genuine MG Rover' short cable doesn't have the square location under the thread to stop it rotating in the lever. Whoever manufactures this part have managed to make a pants design even worst. Congratulations.