Originally Posted by
corporate-wage-slave
I would guess it's to do with differences in Point of Sale between USA and UK. I would just leave it awhile. If a flight has just been moved then there is usually quite a bit of shaking around for a week or so and then availability will typically show up for you. Don't accept an alternative booking until that time.
Thanks for the prompt and sensible advice. Will give it a few days and see how it looks before trying again.