Fixed mortgage contract signed Mon is invalid. Rate used was too low. Surely it has to be honored?

My personal banker put the wrong interest rates in my mortgage contract without realising it. They were rates from only 10 days ago and we all signed it and i thought it was a done deal. He rang today to say he made a mistake and he'd do a new contract at the published current higher interest rate. I said, please can you honour the contract out of goodwill? He hasn't come back to me but surely it's legal and they need to stand by it? Please help it's a big monetary difference!