Thank you. I wasn't expecting that to work because the key did unlock it before, but it just wouldn't open when the button was pressed.
But it did work!
When I looked at the open locker I could see that it is a plastic hook that moves at the press of the button. I'm going to give that and the edges of the locker door a thorough clean to see if that helps prevent further problems.
I read elsewhere that the very hot weather may also have been a contributing factor, making the door expand slightly.
Again, many thanks