@Idolse - I agree it may well have been a style at fault, it was one of those things that I only see happening maybe 1 in 100 books and have just got on with working around it rather than figuring out exactly the root cause was. Which was why if droid3p0 happened to have the original to hand we could perhaps confirm what happened in their case at least.
@droid3p0 - there is a forum dedicated to Sigil here on MobileRead. But the short answer is hit ctrl+enter to create a page break. And to join two pages back together you select the "following" page in the book browser on the left, right-click and choose "Merge" which will join it with the page above it. You can usually easily identify calibre created splits because they will stick out as _000 and _001 type of suffixes - in this case select the _001 file and right-click to merge it so it will be appended within the _000 file above it.