I bought some BitCoins in late 2013 (November/December) using MultiBit v0.5.14 if I recall correctly.
I saved the encrypted Wallet.key file and forgot about it (as did so many others).
Now I am (obviously) trying to get ahold of those BitCoins again, but I am running into the exact same problem as you are.
I am not completely sure the password is correct, ...
Does anyone know the correct method of decrypting this?
It doesn't seem to be encrypted. The comments at the start of the file don't mention any form of encryption. It only mentions a common form of encoding - Base58. Encoding is not encryption so no decryption is needed.
# The Base58 encoded private keys are the same format as
# produced by the ...