The Letting Centre

We asked our landlord/landlady if we could end our tenancy giving one month's notice instead of 2.  We wrote on the 18th July and had neither verbal or written response.  We have today received a letter stating they want to keep to the 2 months in our agreement.  How do we stand on this as it doesn't seem reasonable for them to wait 3 weeks before responding - as we didn't hear anything to the contrary we have now put a deposit on a new property and booked removals etc. ???