I had the same thing myself last month but rang them up and they put it right straight away. They are good with customer service - I've placed a few orders over the years and one went wrong before as well and they were the same then. Very laid back, apologetic and quick to say they'd sort it. They'll get you to return the original back 2nd class but refund the postage once it arrives. As you say at £50 it should be right but speak with them and they'll sort it for you, I'm sure.