Nedbank Dont Honour Contracts

we recently bought a home through Nedbank and asked for a fixed interest rate which Nedbank gave us a fixed offer which we accepted in writing. we then signed the contract and wrote on the contract the fixed rating to be done. since then we have had a variable rate and Nedbank says they wont rectify it. basically it doesn't suit them. to me this is a disgusting way to start a new relationship and i warn possible new clients that they go against their contracts.