When the same thing happened to me (HSBC credit card), my account was immediately credited with the disputed amount while they carried out their investigations, so I didn't have to pay it.
It subsequently turned out to be a legitimate transaction, too, but they just added it back on to the next month's statement with no further fuss or charges.
It turns out that it's quite common for the charges to be found to be legitimate - they'll contact the company that raised the charges and ask for details of the transaction, such as the sales voucher for a face-to-face transaction, or full order details for a mail/phone/internet order, and then let you have the details to see if it jogs your memory.
In my case, it was a company which used a sister company's card terminal to process their charges (a no-no apparently), which is why I hadn't recognised the company name on the statement.
It's odd that they managed to produce and dispatch a new card before getting a simple form over to you, though. Perhaps if the amount had been greater, they'd have acted more swiftly with the forms so they could begin their investigations sooner.