TD5 cooling problem.

Hello.

I am looking for advice on a problem that has just developed with my TD5, it is a 1999 model. It is loosing water, about 1.5litre per 100 miles. There is no evidence of a leak, there is no water in the oil, no unusual pressure in the header tank or bubbling up, no white smoke or anything that would indicate water out of the exhaust. I am no mechanic but have a reasonable knowledge but I cant seem to pinpoint this. Is there a common fault or checks I can make? The car drives well and is making as much power as it always has done. Please assist!

I do know that water can leak from a 300 Tdi from the waterpump gasket and the tell tale are rusty streaks on the engine just below the water pump. Not sure if this is the same for a TD5.