Big Seatwave problem - help, please?!

I listed tickets on Seatwave for Sunday at Wireless Festival a few months back. All listed correctly, face value of £85 each. All good.

I got an email a month or so ago telling me that I'd sold the pair of tickets and I should upload - which I duly did.

Just under 2 weeks ago I got an email from one of their Customer Service agents telling me that I uploaded an incorrect ticket and that I'd listed a two-day ticket. I hadn't. A check of my confirmation email said I'd uploaded a ticket for 14th July (Sunday), which was correct. If I'd seen any other date on that confirmation email, it would have been obvious something was amiss and I would have corrected it.

Now 2 weeks of emails back and forth and Seatwave are refusing to acknowledge their error, laying the blame fully onto me. It's apparently been 'excalated' and the lady I'm dealing with now keeps replying to my emails saying that she will charge me "flat £50 + 25% of the ticket value." as a penalty. I've repeatedly told her she has no authorisation to take any payment from my card.

Communicating with seatwave is along the lines of talking to a brick wall. What can I do about this? Is there anybody I could escalate this to?!

Not only do they want to charge me £50 + 25% of £190, but also I'm stuck with £190 worth of tickets that I haven't marketed for sale because I considered them sold!!