Recover Deleted whats app messages

Hi

I have recently changed my phone from an iPhone 3gs to an iPhone 4. I have deleted some whats app messages from the phone because I will be giving it away.

I failed to backup the old phone before deleting the messages (backed up after) so restoring the new phone from the backup did not recover the deleted messages. I have searched the internet but can't seem to find a solution.

Does anyone know of a way to recover these deleted files?

Thanks