Thanks for the thorough response Ed. What a wonderful resource this forum is!
I don’t think the screws are to blame – I did over-tighten it. Couldn’t resist one extra half turn! The screws aren’t terrible quality, but they’re not the best either. It’s possible a better one wouldn’t have broken.
I like the idea of a series of trials! It makes sense to progress up to the most destructive option. The first thing I’ve tried is cutting away a little material to expose the stem:
I now have my neighbours locking pliers and in a mo will try unscrewing it. I’ll update the post with my progress. If it fails I might buy a bolt extractor and try that as my next option.
I think you’re right about those holes by the way, maybe they are intended for attaching jaws. I’m following Paul’s suggested way of doing it explained here – https://youtu.be/VkTqt-rxQfk?t=1058