Dominic, it's been an interesting exercise with the CM850. Unfortunately with this ECM it looks like it has been evolving since its introduction in 2002, the result is the ECM structure between the initial version and later CM850's like your one is very different, it might as well be a different ECM it is that different.
I think I have resolved the problem with the reading failing at the end for you however we still need to work on checksums for all the variations of how this ECM is structured. The ECM we tested with here is from 2005 which still used the structure the same as the initial 2002 version which is all working fine, I don't know when they changed but yours is the later style which we still need to work on.
Hopefully next week we can get an update done that will allow you to read this ECM successfully.