This is fine, As long as you made it clear it was a continuation of the question and was inside the margins it will be read. The pages are scanned in, so you can't write in the margins as the examiners never see that area of the page, but they see everything else. In future ask for an addition lined paper booklet to continue answers in but it will be marked.
Furthermore, I did this in one of my exams last week and asked the head invigilator if this was ok, she said it was