Think i've got the bottom this missing payment for HMV Vouchers. My old Amex card was saved in the system and i accidentally used that to purchase the £450 worth of HMV Vouchers. Somehow it went through, i got my vouchers and i remember getting an e-mail from Amex saying they had been unable to process a payment as it was trying to go through on an expired card.

I got the vouchers, no payment ever went through, i ordered my mini ipad and if HMV hadn't gone bust, i'd have got it for nothing

It's the only possible explanation.