Assured shorthold tenancy ending, what are my options?

I, along with 2 others, have an assured shorthold tenancy for a period of 6 months which ends on the 7th of July. When we were signing this in January the landlord informed us that she had other people moving in in July, and so with the tenancy coming to an end we have found a new house to move to on 7th July. However, when we asked the landlord what she wanted us to do with our keys at the end of the lease, she said that we had not served her with notice, and would continue to be liable for the rent until we gave her 1 months notice. We acted in reliance of what we were told by the landlord in January, and now find ourselves in the position where we may have to pay rent on 2 properties. Where do we stand?