I started using a fresh container of GlassCast 10 about 10 days ago using about 2/3rds of the contents.

Today, when I went to use the remaining epoxy, I found is was severely crystallised and not pourable.

There has been no contamination with the hardner.

What has gone wrong, or is there a very short shelf life once the container is opened?

Common issue with epoxies like Glasscast, especially if it has been cold. There's nothing really wrong with it, you can reverse the crystallization by heating it up at 50 or 60 degrees for a few hours and it will be good as new.