Viagogo issue.... help quickly please!

I tried selling my Jimmy Carr tickets in April for a gig in May. Unfortunately (and I swear the site changed the date for me as I remember trying to put the correct date in a few times without success) I put it for a date some months in the future, September. I did not notice this and dutifully sent the tickets to the buyer. Viagogo informed me that the tickets were for the wrong gig. I pointed out that the seller had 4 or 5 wks at least to inform viagogo and return the tickets. This was not done and I assume the buyer either used or sold the tickets for April. I have since sent and received several messages from viagogo pointing this out. They eventually said they are trying to investigate why the seller did not return the tickets immediately and tell me to wait for their findings.... whilst also sending me automated emails saying they will be charging me £49 for not fulfilling the correct sale. Today I see there is a pending charge on my bank account that will be taken very soon. I asked my bank to stop it but apparently they can't without me changing my card and the payment will go out of my account very soon.
Please help me deal with these crooks. Time is of the essence!