Given an ultimatum over my hours

[mumof3boys!!]

I have worked for a small company for 8 months. When I first joined them I worked 20 hrs per week, hours were increased 4 months ago to 25 per week.
They have now told me they want me to work 40 hours per week, if I don't I have no job. When I had my interview I was asked about working full time and I said I would go up to 30 hrs per week due to having children.
Do I have no rights because I haven't been there for 12 months?

[Hawkwalk]

what is in your contract?