They scan the full page, and are allowed to mark everything written on the page. Im sure thousands of kids right a little bit outside the lines to squeeze in words, I know I did, so you'll be fine
It'd be really unreasonable for the BOS to instruct the markers to not mark answers which are written beneath the lines - It's rather illogical to expect a student to ask for an extra booklet every time they wanted to write another sentence... And can you imagine how much of a pain it would be to collate?