IIRC blockchain.info's wallet uses BIP39 for the seed, so if you are absolutely sure that you are missing specifically the 12th word in the phrase (position matters, and knowing the absolute position means that it is much easier to brute force), then you can go through the BIP 39 wordlist and brute force your way in. There are only 2048 words used and they can be found here: https://github.com/bitcoin/bips/blob/master/bip-0039/english.txt
That's great, thank you for this! Do you know if this wordlist would be the same for the 2013 wallet? As I know it was updated at one point