Ex PDF text appears on the 'edit PDF' page with boxes round each section of the text. Adding or altering text within these boxes works well but, as soon as one expands this text towards that in the next box, that second box does not move down.
New text then overlaps with that below.
With 'normal typing' the addition of an extra line in one paragraph automatically moves subsequent text down; this does not.
How then does one remove these 'boxes'?
You cannot remove these "boxes". These are groupings that Acrobat determines automatically. Because of the effect you are describing, I assume you are using Adobe Acrobat XI or older. In those versions, you had to manually adjust the rest of the page layout after making a change. In Acrobat DC, the boxes below the one you modify will actually move down automatically.
Keep in mind that Acrobat is not a word processor or a desktop publishing application. It's edit features are very limited, in in general only useful for minor changes. For more dramatic changes you should go back to the original document that was used to generate the PDF file, make the change in e.g. MS Word or InDesign or whatever other application was used, and then convert to PDF again. This is the only reliable way to really get what you want.
Many thanks - actually I am using Acrobat DC so there must be an additional step I have missed. As you suggested one can go to an alternative form to correct - I tried 'Rich Text' which worked.
Personally I am not very conversant with these programmes/syswtems so apologies if I am missing the obvious!