@Serpentine - not explicitly that I can recall - I think I opened the file, switched to code view, selected the text, hit Ctrl+F, replaces the spaces with \s+ (it was multiline expression), hit Tab, found I wasn't on Replace so came on here to whine about that
, flicked back to Sigil, put the replace expression I wanted (of blank), and clicked All (and it "didn't"). Yet repeating those same steps since then it is working, so there must have been some other random click I now can't remember...