Help needed regarding online retailer DVDi.co.uk

Right.

I foolishly placed an order with these muppets when they had some HD DVD's cheap.

I sent them an e-mail to cancel the order and they replied saying this had been done.

Astonishingly got a despatched e-mail 10 days after I placed the order.

Contacted them about this and they said send them back to a PO Box address in E17 London. I did this and obtained a proof of posting two weeks ago.

They have still not refunded me and are not answering any e-mails.

Phoned my credit card company and they can't help me.

What can I do?

Looked everywhere for a telephone number for them with no joy.