How to deal with unauthorized use of copyright material on an IPhone app?

If this was in the US you would need a postal address for the developer and since they haven't responded to your email I would contact Itunes to get that address. Send them a letter (return receipt required) demanding that they either stop using it or contact you to negotiate a fee for using it. If they don't respond or comply it's time to hire an attorney. I'll guess the procedure isn't much different tin the UK.