Many thanks, so presumably the problem is caused by formatting characters that are normally hidden from view but which screw things up, is that right?
If so, is there an easy way to make these hidden characters visible in the normal post window, so they can be removed before causing these formatting problems?
Not sure if it's the same thing, but I tend to paste copied text into Notepad, then copy it from there and paste it into a post here, as I've learned that doing it this way removes the weird effects that sometimes happen, like unwanted text colour changes and massive font sizes seeming to come from nowhere. Would doing this also remove whatever hidden formatting characters are causing lines to break in odd places (I assume so, as I can't recall having seen this in the past)?