However, the United Airlines system did decide to charge me $25 dollars to check it, despite my ticket making it clear that I was entitled to one piece of checked luggage gratis. The assistant said he'd never come across this error before but could do nothing about it as he didn't have supervisor privileges, and his supervisor wasn't on duty. I should claim it back from United at Heathrow, he said.
The United people at Heathrow told me they could do nothing, but advised me to use the refund facility on the website.
The website repeatedly got me to scan and upload the relevant documents with an accompanying narrative, only to freeze when I hit Submit.
On the plus side, today my car died, which did a splendid job of putting the loss of $25 in perspective.
I blame Diana Wynne Jones's travel jinx.