At about 11:30 last night while I was happily stacking blocks we had a power cut. Power came back on straight away, back in to Linux, start Minetest, open world 152, error message. Bugger! The mobs must have been having a private war because the error message said there was "no end of argument".
It was way past my bed time and my poor tired pain beleaguered brain refused to accept such a devastating loss. I opened the 152 world folder and started comparing the files in there with the files in a working world folder, just to see if I could spot any obvious differences – there were two.
map.sqlite.journal 11.3 kb didn't exist in other world folders so I shifted it out of there.
env_meta.txt 0 bytes in working maps this file is 50 bytes, not 0. So I deleted it and copied one in from a working map.
Voila! C'est tout bien!
Still a bit lame compared to some of the pictures I've seen in this forum but I like it.
Hope that helps someone.
Cheers,
Andrew.