You are lucky to get away with that little unpaid overtime. The BBC has had a number of articles on this issue over the years. Punch "unpaid overtime site:bbc.co.uk" into your favourite search engine and be dazzled by the number of hits. A particularly telling article:

http://news.bbc.co.uk/2/hi/business/4273169.stm

At least two Canadian provinces (BC and Ontario) have declared a war on teachers and legislated teachers to do voluntary work, most of which is unpaid and takes place outside of normal business hours. That is unionized teaching. Ununionized teachers often face a lot worse. (I was typically doing 20 to 40 hours per week of unpaid labour while teaching on reserves, but that is a very special circumstance.)