How long can a 15 year old work in a day?

Hello, i'm a 15 year old girl from the uk and have been offered a weekend job at a little country pub near where I live. I will be working as a waistress or preparing salads/washing up in the kitchen. The landlady had no qualms about giving me the job, but told me I had to get a letter from school to confirm the hours that I can work. How many hours a day am I aloud to work in a day? and is working past a certain time at night against the law? How much do you think I will be paid? Its quite funny how I forgot to ask her lol. thank you! <3