Thanks for your response. I didn't know that the private key was formatted like a Bitcoin address (version byte plus checksum bytes at the end). Is it possible to import a raw private key (i.e. a 256 bit random number) or is mandatory to format the key in that way?
Most clients expect WIF but it's not hard to get it in WIF format. Just use the wallet details tab on bitaddress.org. Paste your hex key in there and it'll give you all sorts of formats. Probably best to do this on an offline system or via a live cd where you load bitaddress.org and then disconnect from the net.