My 1998 B2500TD has started loosing/using excessive coolant on both local (4 litres a week) and long distance (1 litre/100km) journeys. There is no sign of leakage, temp remains normal, no sign of water in the oil and only white smoke whilst getting to normal temp.

Probably the start of a head gasket breech. You can buy a block tester at your local parts store for thirty to forty dollars. Follow the insructions in the box, and it will detect exhaust gasses in your coolant. this is a pretty sure way to detect a combustion to cooling system breech. The cat converter may be masking the white smoke from the coolant once it reaches temp.