You can cancel a future transaction by logging onto your account. Remember you will need your user ID, Password and Memorable Details.
Click on the section "your account statement online", and you will see your unprocessed instructions. Click on the transaction you would like to cancel and then click the amend button. From here you will be able to choose delete.
Please note that if the transaction you want to cancel is not shown in the "unprocessed instructions" then we will not be able to cancel it as it's already being processed.