Steve,
The Census number is actually the Army serial number that is painted on the door and rear of the lorry, beginning with letter "H". In 1947 the numbers were changed to the two numbers two letters two numbers type of registration plate.
The different engine number is nothing unusual for military vehicles, although it seems to puzzle the collectors. Having worked in the Army workshops for years, I can tell you that not many vehicles went though their service without an engine change. If your Mat had a major rebuild during its life, that would explain, but in other cases, engines were changed rather than repair, for practical reasons, ie time out of service, breakdown in the field, etc. There were massive resources for overhauling engines on production lines, so old units were returned through the system.