You should check your contract to see how your paid time is defined. If you are paid by the hour, and there is no provision in your contract of employment that says that you must work a certain amount of unpaid overtime, then it is likely that you have the right to be paid for every hour that you work. You should consult your union or take independent legal advice if you are not being paid in full.
However it would be legal for your employer to offer you a contract that includes unpaid time, so long as your total pay (excluding non-contractual overtime) is at least as much as the national minimum wage for the full hours that you work.
For example, as the adult rate National Minimum Wage is £6.08 per hour, the legal minimum for a 7 hour day would be £42.56. If your employer also insists that you be there for another 15 minutes then they must pay you at least £44.07 per day (£6.08 x 7.25 hours).
In addition, the extra time will count towards the 48 hour average limit set by the Working Time Regulations. Most workers can't be made to work more than this unless they want to.
If there's a union where you work, it will be worth checking with them, as trade unions make it a priority to reduce unpaid overtime and to ensure that workers are paid for the hours that they are due. Unions have also been very successful in negotiating issues such as proper breaks and, more recently, payment for on-call time spent at work.
