Well, it is ok to go over the lines you are given, as long as the examiner can read it. So if there is a small gap and another question, don't bother writing it there (unless if you are going to write only little amount).
The lines you are given is enough for you to write an answer, and it also tells you how much you should write. so one line would mean an answer or an answer with brief working out.
However, since the whole page will be scanned (at least inside the borders), you can write in the blank space.