I think you should start with your bank. It was an unauthorised debit - it may well have been a mistake but it was wrong. I'm not sure what they will do but you should tell them immediately and write and confirm (not email).
I have a golden rule about buying high price items - always always always use a Credit Card. The Credit Card company IS responsible for items over £100 and will take steps to put things right. Barclaycard recently refunded me £250 when an ebay deal went wrong, they even back dated the refund to the date I made the purchase so I wouldn't pay any interest.
I suspect your vendor will refund the money and send the correct goods but not quite as quick as you would like.