Log onto your account, then go to order details, if there is no cancel button there, then maybe it has already been processed? Try giving them a call in the morning.
Log in, then click 'view the progress of your orders'.
Alternatively, click 'my account' if you are already logged in. Then choose 'track and trace' - should do the same thing. If that does not work, the contact details are at the foot - call them in the morning.
Send them an email cancelling it. This will cover you under the Distance Selling Regs., guaranteeing a full refund, regardless of what their company cancellation policy may be. You can then follow it up tomorrow with them, if you want to.