Hi, I have been renting a room for business purposes ( commercial lease) since January 2014. I get monthly invoices which I pay and get receipts for. I have now just received a phone call and invoice as the company I rent the room off have been apparently undercharging me each month. They can not locate a copy of the tenancy agreement dated January but they recently sent through another tenancy agreement which I signed and sent back to them. It was within half an hour of receiving this newly signed agreement that they contacted me to inform me of their mistake and tell me that I owe them the money. They are now stating that I am liable to pay the sum of $4,800 that they undercharged me. Do I have to pay this even though they made the mistake. I simply paid each invoice as they sent it. The new agreement does say that the tenancy commenced on 6th Jan but it was signed by me on 30th July. Can anyone help with where I stand under contract law? Thanks in advance.