(under) Payment Legality

sheramber

More likely there was meeting where you agreed to pay the full price.

You must have known how much you had to pay and that they had asked for less. 

You don't not notice £1000 less. 

JetpackVelociraptor

Short answer, yes. Long answer: yeeessssssssssssssssssssssssssssssssssssssssssss

You signed a contract that stated you would pay £22k. You have written email evidence that you paid £21k. You have 0 written email evidence or any other kind of evidence to suggest that the contract was changed to be for £21k at any point. Therefore you clearly and obviously owe an additional £1k. It is an obvious mistake on the part of the employee that sent the email with the payment request. Should they choose to take you to court to recover the money, you will be unable to prove that you do not owe them the additional £1000. There is no mileage in imaging what may possibly could have perhaps happened that would mean you do not owe them this money, because it did not happen and there is no evidence of it having happened.