David mentioned it might be possible to repair the glass. He was thinking of my new camera, but I was thinking of the old one. I had thought of repairing it myself; I had googled around looking for solutions. I had heard of some stuff, clear resin or something, you can smear over scratches, which fills them up without (much) optic distortion. That sounded good! But I couldn't find it. But David mentioned polishing the glass; not adding material, but removing it! Maybe that could work. He mentioned a Dremel for the purpose. I have one! And he said he had some polishing liquid. It was well worth the try. I had very little to lose!
By sheer coincidence I found a left-over glass cover for a microslide when I was making a new reference collection. It was clear glass with a matt side you could write on. Perfect! I could see if I could polish the matt bit, while having the smooth bit as a test of whether the polishing would actually make things worse. I set to work!
The results were encouraging! The smooth glass wasn't affected but the matt bit became decidedly less matt. I decided to give it a go on the camera! I polished it, shoved a battery in, noticed it still had a memory card, and gave it a go. It could focus again! And it will never be as good as it was when it was brand new but I'm taking that camera with me next time I go underground. Hurray for keeping a camera going for a long time under trying circumstances!
The glass cover slide with its part-polished matt bit
The old camera focusses again!