You are legally entitled to a 20-minute break if you are on a Stint which is 6 hours or longer. There may not be a particular member of staff watching over you (therefore monitoring how many hours you have been working) - so you might need to ask for one.
If you are on a 6-hour or more Stint and you have been working for a few hours, you should ask a member of staff if you can take your break - never go for a break without asking or letting someone know first. For health and safety reasons, the business needs to know where you are at all times.
Top tip: Ask a member of staff at the start of your Stint when is the best time to go for a break later on.