Many thanks again. As you will have gathered, I really didn't understand what Lightroom does with the adjustments but obviously you are both right. My back-ups do have sidecar .xmp files but these don't contain the adjustments only in-camera metadata.
I have now been through all my subsequent edits not affected by this problem and manually saved the adjustments to disk in the .xmp sidecar files. I have also now found the option to turn on automatically saving the adjustments to those. Since my back-ups already pick up the .xmp files I should be covered in future, though I shall certainly be re-educating myself to get away from the old folder mentality asap.
Rob is quite right, re-editing 210 images is a pain but it could have been so much worse. Time to get on with that job. Thank again both of you.