Bought a rear silencer box and pipe (without the middle silencer bubble) but the diameter of the flared joining end is too small, even though it was purchased from a reputable factor using my reg. no. Does anyone have the Fiat part No. for this item, Fits a Seicento 2003 Hatchback 1.1 petrol?

Best bet is to have a look on ePER (link in the blue banner at the top of the pages), then navigate to the correct section:

Fiat » SEICENTO » 1E SEICENTO (1998-2010) » 102 FUEL INJECTION SYSTEM » 10258 EXHAUST PIPING

In there should be all permutations for all models.

Hope this helps.