My landlord is trying to rip me off, can anyone help?

I signed a contract which was drawn up by the landlord stating the basics, how much i owed her, the tenancy period (6mths) and if i decide to leave i need to give a months notice, no other terms or conditions were added. Then after two months I decided to return to NZ, I gave her my notice, 3 weeks, short a week but I told her I was prepared to pay her for the extra week. I had paid her rent up until the vacating date, as is normal practice. She returned my bond no problem, I left the UK, then I sent her an email asking her about returning the remainder of my rent, she said she wasn't going to give it back because I had told her that she could keep it, then she said that I led her to believe that I was staying long term, but after I had given her my notice she said she didn't expect anyone to stay on forever and that people's cirstances change, she also said that i never cleaned which is untrue but because of that she will be putting my rent towards getting a cleaner (over �400). Not once did she mention having a problem with my so called lack of cleaning during the two months. My friend says I should just walk away from it but I'm annoyed that she's taking advantage of the fact that I'm out of the country. Is there anyone out there who can help or give my me some advice?