Who do you contact, when your beeing underpaid by your employer, is there an ombudsman that can help me?

I'm self employed and work through an agency. I feel that my hours worked and hours paid do not tally. I have kept a personal record of my hours, as I was told a time sheet would be sent out. (Which never arrived) the agency are refusing to pay me the hours I have worked, and are just paying me a flat 8hr day. Is there someone who can help me challenge the agency?