As recounted later, the Argos ladies sorted that out pretty well.
But yesterday, after weeks of use, the chair developed another fault - the back had swivelled round. I straightened it, but only later did I discover a machine screw had fallen out of it and onto the carpet! At first I thought it was the wrong size screw, but after nearly an hour of wrestling with it this morning, and much cursing, I got it back together again.
I have to go out now, and only time will tell whether I got lucky, or the chair is jinxed...