Not being paid for over contracted hours

Hi-

I have been freelancing as a Marketing Manager for the past 6 weeks.

My employer has refused to pay me for any additional hours over my contracted 25 per month.

She says that the were not authorised- even though she was aware of me telling her that I am over my hours and required immediate work on certain days outside regular office hours.

I am not charging overtime- just the normal pay for the hours.

Do I have any rights to claim for those hours?

Any help is greatly appreciated!!!!!!!