A favorite trick for getting a proper grip on these cords with BS1363/BS546 15A plugs was to wrap several turns of insulation tape around the cord to increase its diameter.
Still widely used here to get zip cord or NISPT into a Schuko plug, sometimes the cord grips don't even grip 3x1mm2 round flex properly. I just had to do that trick on a trailing sockets a few months ago.
By the way, the fancy way to keep the trimmed cotton ends of these cords neat and tidy for termination is to apply heatshrink, but the average person just uses the tape trick again.
Actually there's an even more sophisticated (and much older) way to do that. (quote from my famous 1958 DIY book, covering everything, from painting, glazing, carpentry to plumbing, electrical and cleaning your gas boiler).
Take some black thread, wrap it nicely around the cord end and tie it into a knot. Then apply some glue.
I'll try to scan the picture that came along with it until tomorrow.
The new cloth flex matches the old one perfectly except for the color code. Well, the old one was usually pretty yellowed, but the black/white zigzag pattern looks perfectly the same. About 2 years ago I replaced the cord of my mom's infrared lamp, and you can't tell the difference, apart from the half dozen or so tape wraps being gone.
I'd really like to get hold of some of the old twisted cord for a reasonable price to build a small example knob&tube wiring. I already got a matching switch and light fixture (a huge wet rated porcelaine socket and a round white glass shade, I took it down when we rewired our basement).
