Thanks everyone, I've learned a lot from your replies. Although I knew the threads were rolled, not die cut I never appreciated why. Now you've explained, it all makes perfect sense! Thanks for your patience with a bewildered old duffer.
Ironically I had the chance to buy an old spoke rolling and cutting machine some months ago but it seemed a bit expensive for a worn piece of kit and I didn't think I'd ever need one. I'll see if I can contact the guy who bought it and see if he can help me out or even lend it to me for a day. Most of the LBS's left in business around here don't even build wheels, just try to sell new wheels from a catalogue or persuade you to buy a new bike. One offered to order me new spokes at £2.00 Sterling per spoke plus 55 pence per nipple - probably about $4 each! This makes adapting my cheap ones worth while. However there are one or two 'real' bike shops in outlying villages that might be worth a phone call.........
Once again, thanks for your help!