Its actually
www.acas.org.uk
By the amount of questions you have submitted so far amazingtonic, I suggest you either get some serious legal advice from a CAB or Law Centre ....or get a new job
If you signed terms and conditions to work 20 hours, any variation in hours is subject to agreement between you and your employer .
Your employer does not however have to agree to your request to vary your hours.
It also works the other way around although if you refuse to agree to vary your hours, after consultation, then your only remedy in that case is to resign and claim breach of contract at an Employment Tribunal.
You can also phone the Acas helpline 0845 47 47 47