@YANC
"Are you saying that every university or startup that doesn't have signed timesheets is breaking the law?"
Judging by the responses so far, I'm saying people are clearly unaware of their rights, are not exercising those rights, and are being walked all over by their employers.
If your employer makes you sign a 48 hour opt out, the law says that is illegal.
If your employer makes you work more than 48 hours a week without opting out and doesn't pay any overtime, that is breaking 2 laws. First is the working time regulations, the second is the minimum wage laws.
If you willingly sign the opt-out, and your employer repeatedly forces you to work for example an hour per day additional work at no pay, the company is breaking minimum wage laws. (The law actually considers the weekly average overtime worked, with this based on all additional hours worked over a 17 week period, then averaged out. This then provides the basis for the minimum wage claim).
If you willingly sign the opt out, and work a 14 hour shift fully paid, and then another the day after, the company is breaking the working time directive by allowing you to exceed your statutory maximum. Opt-outs don't apply.
If you are under 18 and are working a 48 hour week, the company is breaking the law because under 18s cannot work more than 40 hours a week, and they cannot opt out.
In all these cases if the HSE investigate, and the company has not got any documentation that can disprove the claims of the employee, then the company has broken the law by default and will get the appropriate slap on the wrist. Like most corporate crime, if no investigation is made, then no laws have been broken. Employers and HR departments walk a fine line when they mislead employees about their rights, and many of these companies are aware they will get away with it most of the time because people are sheep, the comments in this thread back that up. People don't read the laws that give them rights.
Don't take my word for it, google is your friend. Or click the links I posted above which takes you to the relevant regulations, and pages upon pages of explanations.
*There are some exclusions and additions to these regs like the armed forces etc but these are specific cases that have been written into law for that profession.