You could also check to see whether "Don't add space between lines of the same style" is checked in the paragraph formatting dialogue.

Thanks for your response.
The sections of text are definitely separate paragraphs, this is something I'd previously checked out, along with making sure the box was left unticked for "Don't add space between lines of the same style"..... The only other difference I have just actually noticed is that the default set for line spacing is 1.2. When you select the 'Line and Paragraph Spacing' drop down box, there is no tick against any of the line spacing options listed, however when I go in and select the Line Spacing Options, the default in the 'At' drop down box is 1.2... I hope that makes sense. Do you think the default in the 'At' box needs to be set at 1? or something else in the default needs to be reset.
And yes F4 is working for other simple actions.