Thanks for the help/encouragement/commiserations ladies. I unpicked the lot, bought wide elastic from the rip off supermarket down the road ( 5 euros for a packaged metre !) and put that in instead. I don’t like the look as much as the row of narrower channels, but nervous breakdown was looming! The fabric was deceptive. It’s actually silk, which sounds slippery, yes? Parts of the weave are satin, parts not, and those parts had a tendency to have a slub or two, and stick to each other and the elastic, so even with the wide stuff and an even wider channel the gathering didn’t readily even up.
It’s surprising how many hiccups there can be in the simplest of processes we put ourselves through when we sew isn’t it? Threading elastic is hardly up there with bound buttonholes, turning rever points and pocket welts, but I reckon I could write a book on the ways it can have you tearing your hair out. Or is it just me?