Hi Adam,
My apologies for the delay in getting back to you - I am not usually very active on the forums.
Coming to your issue, I'd recommend you update your version of SPIFlash to the latest version - 2.2.0. It has an example called 'getAddressEx.ino' which should walk you through the process. The latest version also simplifies the getAddress() function when it comes to Strings.
If it still doesn't work, feel free to let me know here - I'll keep an eye on this thread for your reply. 
Cheers,
Praj