Decrypting files with an unknown method but a known result I am assuming there is a very simple way of working this out by brute force, but I am not sure if there is a better way.

I have a file of data that I wish to get (my data, generated by a machine). This file is encrypted by some method that I have no details of (and the company has folded).
However, as well as the encrypted file I have an unencrypted file by the same machine for a different date range. Looking at the file it is easy to see that the encryption is in blocks of 8 bytes, as most of the header is zeros, so where the new file says 00 00 00 00 00 00 00 00 the old file says B5 B4 24 EF 45 BC 68 C5.

So I know what 8 0s are like, and 8 ffs and a few other items. I also know it is a block as a an area that is 7 0s and one digit is completely different.

Do I just assume it is RSA or something and make a brute force on the password?
No, don't assume it is RSA as it likely isn't based on the information.

You already have a possible block length. 64 bit block length could mean DES (or one of its variants such as 3DES) or blowfish. Another possibilty is that there was an 8 byte pad used in repeated XOR fashion. I'm sure there are other possible block ciphers with 8 byte block size.

Next I would look for repeated blocks. Anywhere in the plaintext file where the data is the same, see if the ciphertext data is the same. This would suggest either repeated XOR or ECB mode. If this is not the case, other modes (CBC, etc) are likely. The absence of repeated cipher blocks (where there are duplicate plaintext blocks) would seem to eliminate repeated XOR.

If that is all the information you have, your next step would be to brute force. If repeated XOR is a possibility, and since you know the header is 00 00 ..., the ciphertext would be your XOR pad. So try that and see if the decrypted file looks as you would expect.

Otherwise, I'd start with DES as it is probably the more popular of the two ciphers.