I've got the cruise working again. I'd pushed the plunger in! Didn't know what I'd done. Went out, took switch out, pulled plunger, clickety-click, 'Aah!' Put it back. Bingo. Brake lights not stuck on anymore.
Which is strange because the same seller has the same switch listed at £14.95. When I've checked to see if resetting the existing switch has done anything for the ABS, I can always try the new one. If that doesn't make any difference, I'll have a spare. Thanks for the link to the Rave manual. Yes, it was a Saab. I'm told it was the post-classic GM Saab 900.