The elastic arrived, and it's very nice. One long reel, 25 meters.
So I took the trousers with the failed elastic, and tied the new elastic to one end of the cord that runs around inside it. After considerable tugging, I found that the cord is attached halfway round, to the trousers. So I cut a small hole halfway round to get access to the cord, and cut the cord. And the same on the other side. That meant that I was able to remove the two pieces of cord and, at the same time, thread the elastic.
The outcome is that my £3 trousers are repaired.
It's not just that £3 is such a huge sum that it's worth repairing the trousers, it's more that I get a lot of satisfaction from repairing things.
So now I have about 23 meters of good elastic. Because I have more such trousers, and I'll be able to fix those too.
Or I could make a catapult.