Does the number start with 235. If not, it is probably not a valid MMSI number and someone has just entered a randon number to see what happens. Alternatively, they have entered the number thinking it is correct, confirmed it, and discovered an error and couldn't be ar*ed to return it. Unfortunately, once confirmed on the radio, it cannot be changed without returning to the supplier/manufacturer. If it is a MMSI number, then your recourse, I believe, will be to OFCOM.
Terry B.