Yes of course I see. Should of figured that one out myself.
For aesthetic reasons I wanted both divs to be the same height. Without a declaration one is shorter than the other, which I think doesn't look so good.
The contact from div has no declaration whilst the div next-door does. The difference in height now is about 1px which I can just about live with. The reset.css I used helped plus I learned about browser quirks, which I find very annoying! The contact form div now expands to fit success message.
As a newcomer the site wasn't designed with flexibility and these considerations in mind. Something to think more about when I plan my next site.