Grrr. I've just wasted a morning...
Uploading some ads, I noticed a nifty feature whereby on the first page of creating a new ad, you can copy an existing ad.
"Great!" I thought. That will save me having to re-write the ads as I have a lot of similar products. So, I'm happily copying my ads until I hit a minor snag. I'm using Widget A ad group as my template for Widget B. On looking at the ad text, I notice I've spelt a word wrong in Widget A's ad text. So I finish up my ad for Widget B and head back to Widget A only to find that the ad text for Widget A has now been changed to the ad text for Widget B.
Hoping it's a one off, I head back to check one of my other ads. Exactly the same problem - the template now has the editted text, rather than the original text.
The lesson here is, always double check your work because now I'm going to have to spend a day checking all my ad-groups are OK because I can't remember which ones I used as a template base.