Always mark the answers on the answer paper and use a decent eraser to rub out the mistakes. If the computer thinks you have marked 2 boxes, it will automatically mark it as wrong.
cjuk28 You can always pay for a remark which is done by hand I think, also I would advise you to talk to your CGI. The only thing else you can do is keep up with the GNAV study and get it next time.
Good luck