If Citroen's computer generated V55 (new car registration form) was lost somewhere between them and the dealer, the salesman might have filled in the engine size wrong when doing a manual one.

There are so many special editions and stuff out there that no-one in DVLA would bother to query the 1.4 bit and the car would be registered as that.

HPI then pick up the model information from the DVLA record.

If you are sure it is a 1.6, check that the engine number that is fitted to the car matches, and then just send the logbook back to Swansea with a letter pointing out the error quoting that number and asking for it to be corrected.