Just to add to what I have said above... on my local copy, I have entered negative settings for margins above and below (-5 top, and -11 bottom) and the spaces have disappeared.
Obviously this has fixed the problem aesthetically, but it is still a bodge job and I would like to know why this has happened in the first place. I just can't see why I should need to bodge and I can't see where the spaces have crept in.
Cor CSS is frustrating at times!