I've checked the transformation from the OS projection to the OSM projection and it seems to be correct. The resulting boundary is the right shape and size, but it is shifted to the east. I'm going to check out exactly how much - it would be possible to correct it during the creation of the OSM file.
Why would this data be translated? The transformation could be wrong, the OS data could be wrong, the existing OSM data that I'm comparing it to could be wrong or something else. I have tried the transformation with two separate programs and they both produce the same results. The existing OS data has been gathered by various means and is likely to be mostly right. There is a chance that the OS has an error, I wonder if anyone else has looked at it yet.
If the OS data is OK, then what else have I missed?