When should a cambelt be changed on a 1

I'm not sure that, the info your giving is correct. The Laguna(however the Megane is) isn't available with a 1.9 diesel only a 1.8 and a 3.0 option, if it indeed is a 1.8 the millage is as follows:

72,000 miles (115872km) or 60 months which ever comes first.