Hi l am the 3rd owner of a V reg Seicento with 39000 miles on the clock. I am not sure if it is ever had a cam belt changed and do not know at what mileage one is due. Any idea please ?

the general guideline for a timing belt change is 40,000 miles so it is due now.

60000 usually on timing belt or chain.

good luck

Replacement Interval Guide
Fiat recommend:
1986-12/90: Every 12,000 miles - check & replace if necessary.
Every 36,000 miles - replace.
01/91-04/91: Every 24,000 miles - check & replace if necessary.
Every 60,000 miles - replace.
05/91-98: Every 36,000 miles or 4 years - check & replace if necessary.
Every 63,000 miles - replace.
1999-00: Every 36,000 miles or 4 years - check & replace if necessary.
Every 63,000 miles - replace.
2001 : Every 36,000 miles or 3 years - check & replace if necessary.
Every 72,000 miles or 5 years - replace.
The previous use & service history of the vehicle must always be taken into account.

your timing belt is recommended change is every 60,000 Mile.