Have you been to this site?: http://www.avforums.com/forums/arcam-owners-forum/
I downloaded the 4.12 version a while back and haven't had any problems. There is mention of using this file to go backwards from 5.06 version (which they don't offer as a download) due to problems, so you are probably not alone. I have not had to correct a corrupt firmware problem so I can't help, but I think these guys would be the ones to help. You can also check the release notes, there may be something in there but I couldn't get the file to open for me. Let us know if you solve the problem!
I downloaded the 4.12 version a while back and haven't had any problems. There is mention of using this file to go backwards from 5.06 version (which they don't offer as a download) due to problems, so you are probably not alone. I have not had to correct a corrupt firmware problem so I can't help, but I think these guys would be the ones to help. You can also check the release notes, there may be something in there but I couldn't get the file to open for me. Let us know if you solve the problem!