VIN & V5C
Engine number
Unique identifier stamped on the engine block. Recorded on the V5C; mismatches usually mean a replacement engine, not always fraud.
A change of engine should be reported to the DVLA so the V5C is updated. A car presenting with an engine number different from the V5C and no paperwork explaining the swap is a Major caution — it may be a legitimate but undeclared change, or it may be a stolen engine fitted to disguise its origin.