Write a letter to HR

Job ID: 35782994

Budget: £20 – £250 GBP

My place of work offers contracts of various working hours, the most common being 37 hours, there are also 32 hour, 30 hour and 27.5 hour contracts.
I used to work a lot of overtime to make ends meet but the company is cutting costs and they no longer offer overtime so my basic hours are 27.5 hours a week.
I would like to argue to HR to prove that I deserve a 37 hour per week contract, and won't give up until I have one.