Thanks for the reply TheDucks,
I deleted the TOC (is that where the error messages are pointing?). I noticed that the offending chapters ALL have heavy fonts in h1 and h2 - inconsistent with the OK chapters, so, I got up blank HTML pages (sigil) and copied only the 'text' of each file before re-typing and setting the chapters and headers (h1, h2). Then I deleted the naughty files and renamed the new ones.
I re-generated a new TOC and guess what? It's all the same - those offending (now new) files STILL have the inconsistent LARGE fonts.
There's gotta be a ghost in there and he's driving me CRAZY 'cos I don't know what else to do!!