PAS rack help please?

[SLB]

MOT pointed out slight play in one of the track rod arm joints onto the steering rack ( under the gaiter). It's a long story, but basically I had previously completely stripped down the rack and put back together this joint and lock nut as they were previously set, including a pin in the drilled hole to stop them coming undone. Now back on the car, I undid this joint to take out the play, but in doing so have damaged the thread on the lock nut so it won't do up tight anymore. It appears to be a 20UN thread but on a 21mm shaft - quite odd. Does anyone have a spare lock nut, or failing that the other bit which screws onto the end of the rack which I can slice up to make a lock nut? It's the usual Cortina Mk4? PAS rack.

So after 4 years in the garage, the car was back on the road for 4 days before I broke it. *****.